What makes him...?

They are just living up to the expectation that women give them; that only men cheat and almost all are unfaithful.

Obviously not all men are like that, only a minority, same as women. But my question is specifically about what motivates men who cheat, and I guess whether their motivations are any different to women that do

Perhaps it stems from insecurities of their own. A fear that their existing girlfriend will leave them or cheat on them that they feel the necessity to have somewhat of a 'backup' to help boost their own ego. Of course, this may be the thought process of a very small amount of both men and women. I hope this helps.

It was a joke. Some people do it because they're unhappy in their relationship, but won't breakup with their current partner as they still feel something for them or don't want to hurt them.

Thank you, that makes sense! But are their egos (referring to both men and women here!) So fragile that the importance of validation overrides that of their partners' emotions? Are people really that selfish and if so, was there even any love to begin with?

Do you think it's because they're scoping whether there's a better prize out there and keep the partner as a safety blanket?

And what about men that aren't in relationships (the so-called players, lotharios, casanovas) is that just the result of a raging sex drive? Or are their deeper emotions behind that behavioural pattern?

For some, that could be true but not for others. It really is a very subjective and people have different motives behind their actions.

Yeah, a lot will have high sex drives, but again, it could stem from insecurities and how they want the love and attention, but have too low self esteem to think they could have a relationship. Subjectiveness applies to this too.

Also, can you please stop just saying mean? Because women are just as bad.

I did say in my previous post that both sexes are just as bad when it comes to this, but that I'm specifically trying to understand the motivation behind males' actions and whether it differs between these two genders.

Ok. Well, probably won't vary too much between the genders. All about the person and their ideas/morals

Makes sense, thanks!

What makes men play a woman? Talking to/dating other women, being shady about it. Calling them 'friends' etc.

Where does it stem from? What is the thought process/emotions behind it?