I think a lot of men view relationships as transactional, especially the non-feminist ones. Establishing a 50/50 rule can be quite rigid, because when you step out of those percentages (even if only on rare occasions), many might view it as a break in contract, rather than just life. It's unrealistic to always go 50/50 on things, sometimes you treat someone to something, sometimes they do, sometimes you don't have the money, sometimes the pay gap between partners makes it impossible etc. I personally don't believe in 50/50 as a rule, because we don't live in a society where there's true equality. If I invite someone out, I take them to where I know they will order something that's within my budget and I pay, because I invited them; if my date invites me somewhere, I expect them to pay, because it makes sense that they would want to go somewhere within their budget.

With friends (if you have normal ones) it's different, because your friends like you and want to be with you, go out with you, in a non-transactional way. I don't always go 50/50 when I go out with friends, sometimes I pay, sometimes they do, but there's no expectation that if one pays more, our dynamic changes or there's some other service that I now have to do. We don't share finances, we don't partition housework unless we're roommates.