At my clinic, the norm is no sex during the 2ww, and then it's perfectly okay and healthy. Technically, after the embryo implants, our pregnancies are no different from natural pregnancies.
Each case is different. I, too, don't see why some docs would tell their patients to avoid sex during 12 weeks if they don't have any medical antecedents that put them at risk already. Ever heard of a doctor giving that kind of advice to a woman who became pregnant naturally? Then why give that advice to IVFers?? The progesterone we take puts us above the levels of natural pregnancies, and that extra already gives us additional protection against miscarriage...
If natural, healthy sexual relations cause a miscarriage in a healthy IVFer with no medical history of unexplained miscarriage, then that pregnancy was doomed anyway... Women who became naturally pregnant are far less cautious than IVFers and their m/c stats don't differ from ours (at least in my country). So...
1st and 2nd IVF = BFN 1st FET BFP! m/c at 7 weeks. 2nd FET BFP! 3rd FET BFN Now let's spend the rest of our lives having fun together!