Jeez Julie, talk about a scary hypo! What are you trying to do, scare the folks out of playing....
Actually, the legal and ethical permutations here are quite interesting.
As a general rule, if we are talking about a married couple, most state's have laws that say children born during the term of a marriage are presumptively the husband's. Over time, if the husband takes on the "role" of father (thereby supporting this legal presumption), then he might never be able to avoid his status as "father" in a later divorce action, and therefore be obligated to pay child support just as if the child were his own even with proof that paternity is not his (because of the prior swing encounter).
If the couple is not married, then things get more interesting, because the legal presumption doesn't kick in because the parties chose not to avail themselves of the benefits and protections of the institution of marriage. So let me run with this hypo: couple plays and gal gets pregnant from other guy. They decide to keep the child as their own, and over a course of years raise the child as the parents. They later break-up, and gal gets custody of child and wants child support. Guy counters, "not my child," and of course, paternity test confirms the same. Generally, a court would not award child support to this mother. If she trys to chase the "real father" for child support to help her raise this child, this guy can argue that she is "estopped" from collecting any child support from him because he was left in the dark by her concerning paternity when she chose to raise the child with the other guy who was her boyfriend.
Now these are just two tangental hypos stemming from the original post, and they only discuss a couple of legal angles, and don't even touch upon the interpersonal ethical issues at play in Julie's hypo.
In commitment to greater thought and philosophy,
Yours, RND