unless your doctor has told you not to then you should not have a problem.
i had twins and i had sex up until i was put on full bedrest at 24 weeks. my ob never told me i couldn't or that it was more dangerous. granted once i hit the 20 week mark it was extremely uncomfortable b/c of my enormous belly.
unless you have risk factors for complications intercourse should be ok. If you are unsure, ask your ob. There are several women on here who have had or are having twins and have engaged in intercourse throughout their pregnancies.
I have never heard that. The only thing I can think of is later in pregnancy because Multiples are higher risk but even then your doc will advice you. if Your doc hasn't warned against it i'd say go for it!!:)