I think maybe the show thought that line to Lindsey "Have you told him?" was meant to get people talking more than it has, but at the end of the day it has to be just the thing about lying or the book getting dropped or something, because really a pregnancy would just be totally tacky.

People saying it's tacky if Lucas elopes with Peyton. Yes it is, but he's also taking a huge gamble after what he did, to put his feelings on the line purely on that moment they shared in the episode and ask for her to marry him. It's an incredible gesture of faith if he does. Tacky or not it doesn't really matter, the material side of things is not LP. If they're married at a bus stop or a big chapel, it will matter that they're married and nothing else.