On Tuesday night my boyfriend was out with one of his female friends, they ended up back at her apartment later that night, both drunk, and ending up making out. The next day he told me, and I really don't know how to react. We have been together a year, but only officially dating since the summer. Both of us have been pretty promiscuous in the past, and this is the first relationship in a long time for both of us, but I have no desire to cheat on him, and was never suspicious that he might on me. Both of us ended up crying, with him apologizing and reassuring me there is no one else he loves or would want to be in a relationship with. He just graduated college, and I feel as though he is having a hard time adjusting, and maybe this is something he just had to get out of his system, or was a test of our relationship.
I really do love him, and have been so happy with him, but now I have so many doubts. How will I be able to trust him? I don't want to be paranoid, but he has a lot of female friends, and still intends to be friends with the girl he kissed. Is this a sign he's really not ready to be monogamous/ in a committed relationship, or now that he's done it and realizes we both feel awful, will this be lesson learned?

I would let this one go. But if something else happens break up with him. If you think you can trust him again then try dating him again.

It's up to you. If he really loves you, you can forgive him. If not, Go Away!

If they were both drunk, then I would let it slide this time.

Remember, the brain acts and reacts differently when drunk. You also said he cried and apologized to you, saying there is no one else he would want to be with? I think he still loves you a lot and just made a silly mistake; but next time if it happens again I'd probably break up with him.

I'm on the line with this one relationships are like gambling. Play your cards right and you'll be happy play them wrong and your heart will shatter.
Think about it do you want to bet on this guy or fold.

Be thankfull for everything good he has done for you and forgive him for this mistake