I've never been a conflictual person, but I discovered my temper when I met this guy. We tried to have a casual relationship, but we are too alike or too different cause we argue/discuss/debate constantly; but we haven't been able to stop having sex. Its like arguing with each other is a turn on. Is this normal?

I am 18 and so is my girlfriend. We haven't had sex, but everything else leading up to it. The reason why we can't do it puzzles me. She is too tight, my two fingers hardly go in there let alone my penis. Help. How/what can I do?