[Elizabeth]

If you sit on his hips without clothing on, there is a very very slight chance. The precum would have to have enough to travel up a leg and into you.
If you sit on his hips with clothing on, there is a next to nothing chance. Sperm doesn't go through clothing too well. If you were wearing jeans or something, you should be fine.
When condoms go on is different for everyone. Some boyfriend like a little foreplay and so sometimes the condom doesn't go on until later. Personally it doesn't go on until we he is about to dive in.
I think that having a plan B pill is a good backup. HOWEVER, do NOT depend on it for birth control. Also, make sure you don't take it too often. I would only use it is a condom broke or it slipped off and I had unprotected sex.