Insurance pregnancy pre-exsisting condition?Discrimination?

I don't understand why and if the federal laws prohibit pregnancy as a preexisting condition but insurance companies can anyway.My boyfriend and I live together have a child and one on the way and I can't get covered on his insurance not because of Aetna but because of UPS.So if we get married really fast and although I am already pregnant will it be considered preexisting or will there be a wait period?Yet UPS allows same couples to get on each others!Doesn't make sense does it?Can we file as a domestic partnership and get the benefits that way?Plus I can't get group insurance to cover me,or individual and istant programs only pay a small amount.I am confused so if you have definite answers and can help I would appreciate your opinion.