Why is it accurate to say that gene flow is largely random with respect to fitness?

I think that it is because of the thought of "who knows if the person you are in love with works out?" I mean, he'll probably tell you if he does, but there's no knowing who you're going to continue the gene flow with. For all you know, it could be a person who works out 24/7 (not literally) or someone who doesn't even go out to jog unless it involves Pokemon Go. See what I mean?

I hope this helps, please make me Brainliest, and have a fun night! :D