I simply don't have the mental energy to give him that. Does that make me a shitty human being?

Five months ago I began dating a girl with a kid. I didn't think it would get that serious. But she turned out to be an amazing human being. Now she wants to move in with me, and that makes me incredibly nervous for a number of reasons: 1) I've never really lived full time with a girl before, 2) we would be leaving our respective apartments and moving into one; so if the relationship sours, I have no easy/convenient escape plan, 3) every now and then, my dick/brain craves other women still, and 4) she has a kid that is incredibly needy.

I think there are alternatives to #2, so let's scratch that one out. As for #1, I think that is the result of other factors, so let's put that one aside.

That leaves me with #3 and #4. Is it normal to be in a five month relationship and still be curious about other women? If I see a hot girl on a bus, I mentally undress her. I believe I love my girlfriend. I can honestly say she's one of the best human beings I've ever met. But I often bring myself back down to earth and remind myself that love is not a rarity. It's EASY for me to find love and get a girl to fall in love with me. Love, in my mind, is merely a product of reproduction and survival. Without it, our ancestors would not have stuck by our women; they'd have abandoned our women and left them to their own devices to raise the offspring and therefore greatly reduced the offspring's odds of survival. But after a few years, the man falls out of love and moves on to the next woman to diversify the gene pool. Rinse and repeat. At least, that is my theory. Am I crazy to think like that or not? After all, there is ample evidence suggesting that love lasts ~2 years. And if I am correct, then what am I to do? Not move in with my gf in anticipation of another failed relationship? For the record, since my last serious relationship (of 1.75 years), I've been through 4 different girls, all of which lasted 2-4 months. I could also play the devil's advocate and say I just haven't met the right girl.

As for her kid... While I like him, he takes a lot of time from her and I. I can't spend the quality time I truly want with her. In fact, I don't care to spend any time with him, and actually try to avoid him since he always wants my attention and to play with me, and I simply don't have the mental energy to give him that. Does that make me a shitty human being? And more importantly, does that mean I should not even be with my gf? She has told me that it's fine, and she doesn't expect me to become a fatherly figure since he already has his dad. But I need others' opinions. And also, does that mean I should not attempt to move in with them? (She has shared custody with her ex.) She has already assured me I'd have my own office/mancave of which no one but her and I would be allowed in. Another question I've just thought about: is issue #4, her kid, truly an issue, or is it a product of #3?