<human>: So I've been in romantic relationships ever since I was 17. Each pretty serious and long-lasting (ranging from 6 months to 2.5 years). I've never cheated on anybody and did not want to - thought that immoral. People cheated on me though, but that's in the past.

Right now I'm in my best relationships so far and my current girlfriend is great - I basically have almost nothing to wish for despite some interest differences but I'm uncertain if it matters that much. 

And yet I cannot help thinking about sex with other girls - not relationships, not love, just sex, and just for a change. Every time I see a beauty out on the street or at work or online I start daydreaming about having a sexual intercourse with her.

I have tried masturbating (yeah, *furiously*) in hopes that it would provide me with the experience/emotions I seek, but it's been not much good so far.

I suppose a lot of men would consider this normal (natural polygamy and all that) but the feeling is new to me. What should I do? If I fall for it, will I be able to look her in the eyes or think about myself the same way? Perhaps I should talk to her about this? She's pretty open in this sense (we even thought of doing a threesome someday).

I thought that someone older or with more experience, someone who went through the same, could share a helpful advice with me.

Condense the text into a few key points
<bot>: Great girlfriend, great relationships. Been faithful all my life. Can't help thinking about banging other girls when I see them. What do?