Notice that Jenny avoided Forest Gump for a long time until he was a successful shrimp fisherman. During that avoided time, Jenny was partying, whoring, doing coke, and possibly shooting intravenously. We know that she and Forrest had sex once in Washington DC. Then she gets on a hippy bus and never sees him for quite a while.
It is established in the movie that Jenny was sexually abused by her own father. We later see scenes of Jenny utilizing many men as utilities and tools as she grows older. Any girl child that is abused by her own father is likely to be disconnected emotionally from men as she matures. Most likely, she reverts to a primitive hominid female state in which she mates with as many of the most bad-assed jerks she can find and then passes off their children with a utility-man, in this case Forrest Gump.
Women have made the human race survive for millenia by passing off other men's children to a utility man. Fortunately for men, DNA testing makes this difficult to do now. Unfortunately it also means that unscrupulous women can poke holes in condoms and rape men for their seeds and obtain child support payments. Likely these unscrupulous women will then blow those payments on cocaine, leave their utility child at the mercy of a tattoo biker step dad and continue the cycle of Jenny. This is why men need a pill- or maybe female robots that are infertile.