The sex thing...well, maybe they are not being discreet. Better than having sex in front of the kids.
There is nothing that can be done except perhaps put together a nice letter. "dear XXX, the kids have brought to our attention that they have heard private matters coming from the privacy of your bedroom..." or whatever. Doesn't change then send certified letter. Just because we do not like does not mean we can do anything about it. Talk to the kids and explain to them that the 2 households do things differently. Do not keep the kid away from mo or you are setting yoursel up for a world of hurt..playing right into mom's hands.