welcome to HealthcareMagic
If the doctor has told you to stop i guess she has probably done so because even your Lycogemzy may be containing the necessary amount of folic acid
if not you can ask her the reason for that.
As you said the placenta is low lying in this scan it will change the place upto 38 weeks of pregnancy
so when you do the next scan may be it will be at a different place on higher side.
As far as sex is concerned as long as the placenta is low lying you should avoid intercourse and once the placenta goes highger then you can have sex in the position you find most comfortable and with protection if possible after the 7 month avoid sex and if you really do it use condom strictly.