You see your man talking to a gorgeous woman, and they seem to really be enjoying the conversation, not to mention each other. When you get the chance to ask him about her, he says, “We’re just friends!” 

Remember inWhen Harry Met Sally, Harry explained that men and women couldn’t really be friends? He said that men always wanted to have sex with attractive women, so the friendship is doomed before it gets started.
Harry’s explanation might be slightly exaggerated, but I think it’s pretty much on the mark. Of course, it goes both ways. There are also plenty of women who have male ‘friends’ they would like nothing more than to have as lovers.
There is no way to get accurate statistics about such things as infidelity, but I will tell you that most affairs begin as friendships. Notice that I didn’t say “one-nighters” or “flings.” I said “affairs.” The reason I’m differentiating between affairs and casual sex is that people who genuinely care for each other, as real friends do, don’t normally use each other strictly for sex (but, of course, there are exceptions). Real friends develop deep concern for each other, which can easily go from being an innocent friendship, to infatuation, to a full-blown love affair in a fairly short period of time.
So you think your husband has a platonic friendship with a woman? Don’t let the fact that she’s not Angelina Jolie give you a false sense of security. When people connect on other levels, the looks are not so important. For example, I had a family member who had an affair with a woman who was less attractive, much less intelligent, and much less successful than his wife. Men (and women) with low self-esteem are particularly vulnerable and are easily attracted to people who are less physically appealing, which makes them feel better about themselves.
I think the most dangerous place for the marriage is the workplace. When two people who have a sexual attraction for each other work together, it’s extremely difficult to control because of the repeated contact they have. If they work closely together, it’s a recipe for disaster. It’s almost impossible to control.
If you think I’m trying to scare you, you’re right. I don’t want you to condone a ‘friendship’ which might start out as just a ‘harmless flirtation’ or business contact but end up as something that will damage, or even destroy, your relationship.
When your man tells you, “I’m not attracted to her! There’s nothing going on,” don’t trust it. I used to think that physical attraction was something that was either there, or it wasn’t–that it couldn’t be cultivated. I learned that I was wrong about that.
Years ago, there was a  guy I met through an acquaintance. The only attraction I felt for him was as a friend. However, over a period of months, we went from small talk to brief conversations. From there, we developed trust and friendship.
After a few months, he told me that he wanted to sleep with me. Looking back, I should’ve seen it coming, but I didn’t. Anyway, over a period of several more months, we grew closer, and eventually, I developed an attraction to him and fell for him. 

Okay, I said all that to make this point: I started out with absolutely no attraction to that guy, to being completely attracted to him. You can’t trust the fact that your man isn’t attracted to a woman he calls a friend. He might not be attracted to her now, but after he gets to know her better, an attraction might develop. Or the relationship might be platonic now because she isn’t attracted to him, but if she were to become vulnerable, she might develop an attraction to him. And, of course, the same could be said about your own feelings for an opposite-sex friend.
I’m not going to say that it’s impossible to have a platonic male-female relationship. And I’m NOT saying that if your man has a female ‘friend,’ he’s sleeping with her. But I will say that if your man has a female friend, and especially if she’s attractive, I would not trust it if I were you. Even if she’s also your friend, if he talks to her or sees her without your being there, you have reason to be suspicious. Call me jaded if you want, but I’ve seen it too many times.