Not really sure, but for some reason, guy's seems surprised when they meet someone new and fall in love. They feel guilty as if they have done something wrong.
But throughout history, many great men has proven that the male is capable of dating hundreds of women - all at one time. Look a Solomon, 700 wives and 300 maids.
This is why it is so important for you, when you are dating someone, to stick with one girl. Don't go messing around with other girls that way, because no matter how strong of a relationship is, you are going to fall for another girl.
Many relationships are based on much of the situations that we are in at the current time. For example, it is only when we are cold that we want heat; when it is hot, we want cold.
Normal human response? Well, no. This just proves how we don't like either one of them.
To be in a relationship and have things change, you have realized how you have felt about your girlfriend in the first place - you've been with her three years and haven't proposed to her, you called her bluff, then you started seeing someone else. As there is a saying, "You fool me once, shame on me. You fool me twice, shame on you. But, you fool me three times, shame on us all."
I suggest you not waist your time trying to justify you actions with your girlfriend. Just respect the way that you feel about her and don't feel no more or no less.
Hope this answers your question!
REFERENCE(S)
1. Dr. MP, 2006.
Relationship coaching and love advice