I have been asked to breed for two couples twice. I only did it because they were really healthy and secure in their marriage, I knew them well, and wanted to help them. I'm a pretty successful guy, intelligent, fit, confident and fun(ny), and in one case the wife wanted her baby to have some of my qualities and her husband admired me a lot (we're friends). The other case was a lesbian couple, so less relevant to this crowd.
I have considered this a great honor to help couples start a family and give them a baby. Plus, whatever the legal arrangement is, that baby is my baby, and I want to know they are taken care of by capable and loving parents. In both cases there is enough racial ambiguity that it won't be clear that the baby isn't the "*******'s" and they don't plan to tell the kid (at least at first) that I am the real *******, I am just a "family friend" who comes around.
If anyone is really thinking of this, I would urge you to think about it very seriously. It moves from the realm of fantasy to a real human life once you get into this, and you should think about it the way you think about IVF/AI. Also, in this age of genetic testing, you should pretty much assume that when the baby is old enough, they will find you if they want - even if you aren't in a genetic database (I am), all they need to do is match with one of your close relatives (i.e. if you have a relative who is in a genetic database, which is very likely) and they'll be able to figure out who you are.