let me help you here, if i can. i too am a straight male, married for going on 20 years. as you, i was horrified on how to tell my kids, i kicked the notion around alot before i came out and told them to sit down for a conversation about sex and protection of std s .
before you know it the conversation turned into a full out sex talk, and for me, being my daughter was only 15, son 14, they actually listened to the full weight of what i was saying.
althought my daughter really didnt take it to heart as well as i wanted at least i felt i did what i felt i needed to do.
they need to know when you are ready to talk, there is no real age of when its right. waiting could be the wrong thing, especially at their ages, they are at the sexual age of life.
all in all, it all depends on when your ready to talk about it.
and also you having a child in med school, you are in a unique place, that they may know more than you think, but untill you deceide to open up on this issue, them knowing what you are coping with is all in the dark. they may also give you more support than you think they are able to, be it if only moral support thats enough for me. good luck, take care