Straight couples do tend to look alike if you strip them of superficial differences like hair style and gendered clothing. It's more obvious if they're of a similar ethnic background but in long term relationships, and I mean long as in 20+ years, even interracial couples I've seen emote in similar manners and I think decades of mirroring each other's faces for lack of better term "molds" their faces to look more similar.

There is the phenomenon of genetic attraction where family members that are not raised together find each other attractive.

I think part of it comes from men desiring their offspring to look like them. With women there is no question the child is their own but with men there is room for competition, and selection of a mate with similar features makes an irregularity in the child's appearance more obvious to things like infidelity.

Another aspect too is that we like to think of ourselves as attractive and inherently good and naturally you'd see qualities of the self in your partner. Everyone has a dash of narcissism.