Intercourse is a natural act.
You do not provide enough information to understand why your embarrassed.
You are surmising. You need to ask if intercourse is so important to you; that you cannot even talk to your sister about the sex act
Intercourse is not between just those two people but with all the people with whom they have had intercourse. STD means sexual transmitted disease. Disease is not a condition normal people normally seek nor desire. Be smart ;deal only with virgins in your future.