Whether its a straight, gay or bi relationship it should be mutually satisfying for both of you regardless of who cums first unless your with a selfish loser and in that case they should be dumped.
I don't think you need to explain anything other than this is your first experience with a man and then figure out who you are/what you want/how you feel about it etc after
Extra precautions need to be taken though with gay/bisexual sex as the risks of STD's are statistically a lot higher so always use condoms unless you have both been tested first and are exclusive
Good luck
"Don't ask a question if you can't handle the answer".