Something you should really expect when you're working for an advice line, is to get callers who want to know if their partner is cheating on them. Happens almost every day. Often I say, yes.
Sometimes they want confirmation because they've been suspecting something already and just don't trust themselves, sometimes they want "proof", sometimes they're just being paranoid. There is the rare case where they want to understand what's going wrong in their own relationship, that's causing this to happen, and what they can change in themselves.
How I approach callers like that differs on a case by case basis. I don't really have a set form. Sometimes, I simply tell them what I perceive. "You're asking about your relationship. You're worried he's cheating, right? I'm getting the same impression. This isn't the first time this has happened, either. And it's been going on for a while. He'll more than likely keep doing that." That's when it's clear. Those are the calls that take 2.5 minutes. Sometimes, they want things I can't give them. Names, reasons, solutions that don't involve "If this is unacceptable to you, leave."
One thing I ask every time is, "What do you want?" How crazy are you about this person? What's it worth to you to be with them, how much do you care if they also sleep with others, how open minded are you and where is your heart? Those are the difficult questions, and the only ones that really matter in any situation involving cheating.
I see it as part of my job not to judge. My own personal moral compass and my personal opinion on their life choices isn't what these people are calling me for, nor what they need. I don't see sleeping with other people outside of a relationship as a problem, necessarily. It all depends on the parameters of the relationship. Are you exclusive? Have you discussed this together? If you have come to an agreement before, does it still apply? If you are thinking about cheating but you don't want to because you'd feel too guilty, have you thought of talking to your partner, and upgrading the parameters of your relationship to the current situation? Maybe they are ok with the things you want.
Relationships aren't fixed things. They are not cages in which you trap someone you love. If you are treating your relationship that way, and you think it's perfectly reasonable to keep someone in a cage of being "loyal, devoted and exclusive" no matter what is going on between you or what would make them happy, you need to think again and see if you are providing that other ingredient so important to a relationship: love. Or at least kindness, caring, understanding. People change, and their interactions change.
More than once, I've had callers in a cheating situation who jumped right into the victim position as soon as the notion of "he's cheating" comes up - while completely ignoring the other relevant factors, like "You two haven't had sex together for 5 years because you won't let him touch you," and "You're always yelling at him". People get upset about cheating because they're insecure. They're worried that once their partner is allowed to start considering other people, they will realise that they've stuck a bad deal, and leave for greener pastures. Almost every time when someone calls me about cheating (and remember, this is just about daily), it's a two-sided story. Somewhere in the relationship, both parties stopped talking to each other. They stopped asking "What's going on with you? Are you happy? Do you enjoy being around me? Is there something I can give you that you would like very much, or is there something you need to be happy, that I can't give you? Here is what I need in a relationship these days. It's different from when we started dating all those years ago. Do you think you could give me that? Is there something you need, or want, that I may not know about? Are we still compatible? How can we keep making sure that we're both happy with our being together?"
The problem with cheating is not sleeping with other people. It's the betrayal of trust, the (often mutual) feeling of rejection, and the breaking of an agreement that makes the mess. I know several people who are in committed relationships and who still regularly have sex with others. On purpose. Without guilt or shame or drama and without making their main relationship worse for it. They have an open, honest relationship with their partner, and they have communicated, "I want to have more sex. I want to have sex with different people, in ways that you don't enjoy. I care about you, and I want you in my life, and I want this other thing very much too. Can you work with that, or not?" They've made agreements with their partners. Not "we are only allowed to sleep with each other", but agreements nonetheless. Like "Sure, but tell me about it first, I don't like the feeling of things happening behind me back." Or, "Ok, but I don't want to know about it." Or, "No problem, as long as it's just about sex. If it takes away from what we have together, I want you to talk to me first." Perhaps, "Have at it, and can I watch?" or "If you want to sleep with that other person, go ahead, bring them home, we'll do it together." Or just simply, "Great, but then me too."
It's a negotiation, a give-and-take, an aligning of wants and needs and boundaries from both sides. The fundamental premise is still the same as in any more old-fashioned relationship. It still says, "I care about you, I like having you around, I want you in my life, and I want to be able to trust you. I want you to be happy, and I want myself to be happy." And it works, as long as you have the same ingredients that make any relationship work: communication, caring, trust, honesty, integrity.
So cheating, yes or no? Both. Yes, be yourself. Yes, absolutely and enthusiastically, do the things that give you happiness in your life. If that means having fifteen different lovers, have at it. If having at it would make your partner very upset or sad or scared, talk to them. Explain your needs, listen to theirs, find out if you two are still compatible. If you feel the relationship needs work, work on it. If you have baggage, that's yours to address, not theirs to tiptoe around forever. No, don't cheat because they won't give you what you want. If you can't agree, break it off. If you're stuck in an agreement you can't be happy with, communicate and change the agreement, or leave the relationship. No, don't deny them the things they need to be happy, just because you feel scared and insecure. The relationship won't work if it's based in your insecurity and their unhappiness, anyway.
Let's go back to making relationships about people who care about each other, love to have each other around, and want very much to see themselves and the other person be as happy as possible.