<human>: Can you succinctly summarize the text in 1-3 sentences?

Our relationship is in its 4th year, and is generally going well. Its only my second relationship ever, the first also being long-term. I have only had these two sex partners. She adores me, and is talking about marriage and kids. I'm super confused. She is beautiful, smart and has a wild and insatiable sex drive. All is well. But reddit, why do i keep looking at other girls? I find myself eyeing up every girl i see, mentally undress them, and wonder what sex would be like with them. Every girl i talk to, i compare to my girlfriend - is she funnier, smarter? What would life be like if i was with her instead? 

Its driving me mad. I keep thinking about having missed out on a proper single-life existence with one-night stands and short relationships to learn about myself and what i like in a partner. I've been quite socially awkward most of my life, and have had very minimal interaction with girls. I keep wondering whether my current relationship is *the one*. Because how can i know? If i'm deciding on my favourite type of cake, i want to try more than two.  

One half of me wants to get out there and try things out. Now that i've become fairly outgoing, and with so many online dating possibilities, i think i would have a fairly easy time getting to know many girls. But that means giving up something i also feel is so good. I have a brilliant future wife and future mother of my kids right now. Is it worth giving that up? What if i find out what i have now is indeed the best?

Reddit, is the grass always greener? :(
<bot>: Only 2 relationships, and very little interaction with females throughout my life in general. How do i know if my girlfriend is the one?