I don't think it's tacky. Actually, a lot of my friends on Twitter post when they have sex. Not EVERY time, but when something especially awesome happens:
"Heck yes! Tried blankety blank for the first time and it was so freaking great!"
But then, I follow a lot of kinky/sex accounts so I don't care and I enjoy reading what others get up to.
If it was somewhere my family was, like on Facebook (which I no longer have), then I'd be really annoyed, but, since I'm not on there anymore, I guess I wouldn't know unless my family told me. And believe me, they would.