For the "You'll never get them out of your bed." comment I usually say something like, "Really? Well, I slept with my parents until I was 6 when my brother got old enough to share a bed with me. Then *he* slept with me for a couple years. We both turned out OK, don't have sleep problems, and TRUST ME neither of us has any urge to sleep with our parents or each other."
Kinda wordy, but it works and I've never had anyone argue with me about it because....well...I'm speaking from first hand experience you know?
I've also been known to reply "Do YOU like to sleep alone? No? Then why should my children? It works for us and it's really none of your business anyway." Of course, it helps if you know the person you're talking to well enough to know that they actually *don't* sleep alone each night before you break that reply out.
As for the sex thing for a baby I'd reply something like, "They're in the crib that's attached to the bed & they're asleep. Honestly, they don't know what you're doing. Get over it." For our toddler I say, "She starts the night in her room & comes in with us when we go to sleep. We have plenty of personal time. Besides, it's not like the bedroom is the only place you can do it!"