I'd like to enjoy this pairing a lot more than I actually enjoy it which is a shame. I think they're fun together and strike me as a decent match but I can't get past the ick factor. He remembers her as a kid and was her babysitter, he used to fancy her big sister.
Well, if this is any help:
1) The main reason that Dander can come off as creepy is because Nicholas Brendon, who played Xander, was
10 years older than his character. He was 26 when he started playing the 16-year-old Xander. In the comics, of course, he looks his proper age, so, this should help you get over the ick-factor.
2) Don't forget - when Xander was babysitting Dawn, she was 14-years-old and he was 19-years-old. The fact that Dawn had a babysitter at all in this age is ridiculous, and is a testament to the over-protectiveness of Buffy and Joyce (which is probably partially responsible for Dawn being a little immature for her age in seasons 5 & 6).
3) Xander is just 5 years older than Dawn, and Dawn is over 18 when she and Xander start to grow close romantically. So, this isn't pedophilic at all. If anything, 250-years-old Angel having sex with barely-17-years-old Buffy was a LOT more pedophilic.
4) Xander was always kinda immature for his age, so being with someone slightly younger than him is pretty fitting. If anything, Dawn is more mature than Xander by season 9:
Spoilers for season ten, issue 15:
5) Most of all, if you still have trouble accepting Xander being in a relationship with little baby Dawnie, just look at these for a couple of minutes:
(I put it in spoiler, because pic-spam)
And don't get me wrong - I'm not trying to defend the story, I'm just trying to help you enjoy the story. I had some trouble with the idea of Xander/Dawn too, at first, but I grew to accept and love them a lot over time.