Of course. A thousand decisions lead to the point of the affair, the other person is certainly not the reason for it. Personally, I have had no qualms about engaging in sex with a person I knew or suspected to be in a relationship. I'm the least of their worries. As for the fat part... we see it often, because it's so well documented on Tumblr and Facebook, many let themselves go once they have a committed partner. Women especially, and now women are ahead of men in obesity rates. All else being equal, that means there are lots of human sized husbands out there stuck in marriages with women who did the old bait-and-switch on them. Sometimes bot  7ffa  h give up. I'm assuming either way the result is an empty, depressing relationship and shitty sex life, if any, just keeping up appearances. Then kids are involved, and it's even worse. Hardly surprising divorce rates are so high, and marriage rates are falling too, with so many entitled fat people looking for their fitness model partners they so deserve.