what do I do? cheating boyfriend

Natacha
My boyfriend and I have been happily together 10 years. He is romantic and loves me to death and showcases me off to everyone. He recently has been away for work, 6 weeks. He came back for our first ultrasound and I found out this:
He went clubbing with friends multiple times but hid it from me and met a girl one night which he drove back home and gave her a 'peck on the lips'... says the alcohol is what made him do it... he saw her many times after but only with his friends there and says nothing ever happened after... I know his friends well enough to know they wouldn't let him do anything with another woman, since he's with me... 
What am I supposed to do?
I can't imagine ending my relationship with him, but I also have no more trust