Is this really Happening?

My boyfriend and i have been together for 2 years and have been wanting to get pregnant so bad! well finally we did!!! yay! and we have such a wonderful relationship, or so i thought! a week after we found out i was pregnant, he went out to the bar, got all whiskey drunk and made out with a girl that i hate and he know i hate her! he says he was taken advantage of, but he didnt push her off or say no. My mother says"i know you need him to get through this pregnancy, but i would not marry him!" and now he pressuring me into getting married b4 the baby is born. Am i just setting myself up for more heartache? what should I do? what would you do?