Yes. In fact, heterosexual men are genetically polygamists.
The reasons may include:
-> Lack of satisfaction from sex.
-> Not loving one's spouse/girlfriend/boyfriend.
-> The feeling of freshness from the other person.
-> The other person being prettier than one's partner.
What the **** is this mess?
So if a straight guy cheats he can say "oh but it's in my genes". Sure, Jan.
No. Even if the relationship has deteriorated I think you at least owe it to the other person to be honest about your feelings. If that doesn't help, dump him.