Guys, Is it very likely for a guy to have a girl on the side if or when he is dating someone?

I'm very curious if a guy is dating a girl that's very loyal to him and has woman/wife like features for him to have someone on the side just in case it doesn't work out, and I'm talking about a guy that had 15+ bodies and dated almost the whole county, and he dated a girl he meet in highschool that was his bestfriend forever, she had 2 body counts and is very loyal and sweet.

Guys, Is it very likely for a guy to have a girl on the side if or when he is dating someone?
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