Is it normal to fall for your best friend's step daughter?

I had a stunning 21 year-old claiming she loved me,,, when I was 36. I figured it was an infatuation that'd soon wear off but after 8 months it hadn't. She was as smart and witty as she was gorgeous and I was sorely tempted. BUT, while I wouldn't say dating her woulda been wrong, I felt doing so woulda been selfish of me. It wasn't so much - as mentioned above - things like the fact I'd been dating while she was a toddler and had (vinyl) albums that were older than her,,, it was more present/future stuff. She liked clubbing/dancing,, last time I'd done that was 'pogoing' to The Jam, The Stranglers n suchlike. I still go to gigs but clubs were never my thing. The thing that really clinched it was,,, what if she wanted kids when she was 25 - 30? I think parents have a responsibility to at least see their kids through college,,, and I might not even live that long. Potentially widowing a mother in her mid 30's? Nahh, she'd be better off with somebody her own age. It's a cruel act of nature, programming us males to be attracted to fertile young women but consciously, I think if you have a genuine love or affection for them you have to let 'em go.