It seems that generally the following applies (as I think someone has already pointed out):
For a man, sex with their partner allows them to feel close and connected. For women, closeness and connection must happen to allow them to have sex.
That’s a massive catch 22 but it does imply that both sides have to work at it for things to be a success.
It’s no good the woman expecting connection, love and romance if she’s not prepared to work on the sex side. Similarly, if the sex isn’t there it’s no wonder even the most self disciplined of men will quickly doubt their relationship and start to look elsewhere.
Therefore I’d say that generally, women who have affairs are looking for love, romance and connection. Men are looking for sex.
Of course there are always exceptions to the rule but I’d guess the above picture is fairly accurate for 90% of the population.