Bad thoughts

When does fantasizing become a problem? Is it healthy to think of someone else fulfilling an area that your partner doesn't fill?
Fantasizing doesn't mean you don't love your partner but it shows you what you may be lacking. Excitement?  Comedy? Tenderness?
I've complained about the same thing in the past but it is something that keeps reinforcing my idea that we are not necessarily "meant" to be with one specific person. You choose to be with someone and he chooses to be with you but is there room for change in the future?  If we become stagnant then the bond will sever and it could die. You could loose interest or look for a spark with someone else.
Those things are normal but they get annoying. I'm sick and tired of fantasizing about this guy and I making out. Surely you picked up on what is missing for me. I don't always want to have sex... sometimes making out is all I want but that's not something I can get with my partner so I just think about making out with a friend who obviously likes me. That is where my problem is.... I don't want to think of making out with a friend. I am afraid of being alone with him for extended periods of time because I find him attractive but I don't want to compromise my integrity or my relationship with my boyfriend.
I hate this feeling and at the same time I like it because it makes me 'feel'.