After reading the comments posted, I see a lot of good points raised. I would suggest consulting a lawyer and having a contract drawn up between you and the girls, detailing the specifics of your present and future responsibilities. They probably won't have to adopt the child, but either you or Chase (and eventually Gina) should sign off legally on child support, or sometime in the future you could find yourself getting nailed for it.
As for your future spouse being upset about your having kids with other women, I wouldn't worry about it. Lots of people deal with their spouse's kids from previous every day, just fine. Shoot, my boyfriend has a wonderful son from an ex girlfriend that we see once or twice a week right now. It's a complication I hadn't expected to have to deal with, me not wanting kids and all, but it's working out just fine.
As for the actual conception, well, sex is meant to be fun, too, and it can be frivilous. You have the emotional ties of friendship. I don't have sex w/ someone unless I care for them, and what is friendship but caring? Just have fun with it -- and keep in mind, it'll likely happen more than once. After all, the odds of your friend concieving on the first shot (pun intended) are pretty low. Chase and Gina need to be absolutely sure their relationship won't be affected by this act.
Good luck and best wishes.