<human>: Today I found out that my girlfriend of 1.5 years slept with another male but did not have sex. They did nothing else except cuddle. This occurred two months ago, however, she had lied about the ordeal. 

I was aware that she was staying with a male friend, however, when I asked, she stated that they were staying in separate sleeping arrangements. He messaged her late at night (two months later) and I happened to intercept and saw that she was lying about sleeping in different locations. 

Back story: We have been discussing marriage for the past 6-8 months and this is (was) a serious relationship. However, I informed her that I did not want to be with her after finding out that she did sleep in bed with another guy and was dishonest about it. She is trying to get back with me and apologized for lying, (which she claimed that she lied because she did not want to hurt my feelings). 

I'm in a tossup- do I overlook her mistake and take her back, or do I stick to my guns and drop all contact? It is clear that she is attracted to the other male, however, I am basically a "security net" because I am stable with stable income and wanted to get married. 

Even if I decided to get back together, how do I convince myself to ever trust her again? I have problems trusting people to begin with, so after this incident I'm wary that I would even be able to know if she is being truthful should a similar scenario every arise.

Can you give me the gist of the text in a nutshell?
<bot>: Gf of 1.5 years slept (as in sleep) with another male. Lied about it for two months (until I caught on).