Well I definitely don't see either of them planning to get kids. I think they'd be far to busy with, as you say, defending the earth and just enjoy each other's company. In a way I think both of them have seen too much of the world to voluntarily bring a child into it *shrug*. Also their lives isn't suited for anyone else either.

That being said I don't think it'd be too far off for them to have an "accident" and that in that situation they'd be happy about it.

So what I'm trying (and possibly failing, LOL) to say is that I don't think he'd be totally against it but if he'd have a say he wouldn't plan to have a mini-doctor. Anyway I think Rose's take on the whole baby-deal might be more interesting than the Doctor's. I've read a lot of fics where she's totally into kids and sometimes I find that more unlikely than the Doctor wanting one.