Then talk to a lawyer and get proper custody arrangements and schedules in place. If you don't you're just playing with fire and that's not even taking into consideration his sex life but rather arguments and such over when and how much.
This should all be arranged right now before the baby is a toddler and your ex thinks that he can come and take him whenever he wants. If his mother wants to know why you've asked the courts to have his visits at her home then tell her why. If your baby's interests are your focal point then you're going to have to get over trying to please everyone and just focus on what's best for the baby and if that means telling your MIL your fears, then so be it.
I think you should talk to your ex about keeping new women from coming in and out of his young life though. It's just a better upbringing if he does that. It's not fair to a little one to be bonding with lots of different women only to lose them for yet another one. Very confusing and emotionally wrenching not to mention it will teach him that women are disposable commodities that one shouldn't keep around for long.
Last edited by Wakeup; 05-04-13 at 12:51 PM.
Reason: changed the word "should" to "shouldn't"
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion