Is it fair to my boyfriend for my to think of being with another guy?

I love my boyfriend very much but we have had our ups and downs like every couple in a serious relationship does over the years. The problem is that I really like this other guy who I've known for a few years and he knows I like him and I know he likes me too.
I feel like being with my boyfriend has caused me to miss out on meeting new people and seeing what might happen with this other guy.
Is it fair to my boyfriend to be having these thoughts? What should I do?