Again very 2-D. Does a parent doing something for a child that makes the parent feel good a selfish act. The fallacy in your narrative is that there is something wrong with genesis of a relationship being the benefits it provide one side or the other. You are neglecting the possibility that out of that gratification comes a true commitment to serve the other person. That evolution is moving element and is part of the dynamic of a good relationship. What you are railing against at your core is the point in a relationship where that transition isn't made.
having a two parent one home family is better for the child but doesn't necessarily make the child feel better. that child could still have both parents in its life without them married and would never personally know the difference.
what i'm saying is its easy to serve the other person when that interest aligns with your own self interest or its convenient for you to do so. its easy to do that act and tell yourself you did it for the other's sake when it actually served your own purposes as well.
given a situation where the result is beneficial to one party and the other party gains nothing or even loses in the process that is the only way to remove the influence of selfish interest.
its much easier to take a bullet for your best friend than it is to take one for your mortal enemy who you may want to see dead but know they shouldn't be so.
for many people that selfish self interest is never tested. their convenient acts of goodwill go to those they have this emotional bond with and they can hid their selfish interest behind the bond but deep down those acts also serve a selfish purpose.
if i say you should date and marry someone you dislike and find unattractive but they like you and find you attractive your initial reaction is probably gonna be why would i do that? the selfless act of dating and marrying that person may bring them great joy but when it directly conflicts with your own selfish self interest and goes against that you'd probably be inclined to decline.