A bit on the side can be perfectly acceptable. Plenty of people are mature enough to understand sex as sex and relationships as something entirely different. It is entirely possible to have a relationship but also have sex outside of the relationship with a partner's consent. Many people do in fact do this.

As for casual sex relationships. Yes they can be completely devoid of relationship feelings, there are many people like this. Everyone here has probably met plenty of people that they know they would NEVER want a relationship with because of x character defect/s but are still physically attracted to. A relationship takes more than a physical attraction and sometimes that just isn't there, that does not however mean that a physical relationship isn't out of the question. This is absolutely no different for women. There are many different types of people though, some are capable of it, others are not.

You Genepoole are simply of the group that always wants relationships. There are people that want no relationship whatsoever. There are people that enjoy both. There are people that enjoy relationships with sex on the side. There are also people that enjoy polyamory, 3 or more having a single relationship together.

People are different. Not everyone pursues or even wants the same thing. There is no right or wrong in any of this beyond that of the ethical one, which everyone agrees on.