Other then romancing one of them or killing one of them? No...
Then again them hooking up, while feeling a bit out of the blue, isn't that problematic, at least to me (and my main Sheps romanced Tali and Garrus respectively, so...), it's no big deal.
What I find curious however, is that they are literally the only two squadmates that hook up if you don't romance them (there's Jacob too, but he hooks up with someone else regardless of romance, so he doesn't count)...
And they automatically hook up if you start playing in ME3 (you have to import to romance them)...
*Dons tinfoil hat*
This is obviously a evil scheme to condition new players against romancing either of them!
They'd have started seeing them hooking up and thus be unwilling to "break them up" if they go back to previous games!
This in turn will cause the percentage of talimancers and garrusmancers among the fanbase to drop! They want us destroyed, brothers and sisters! *shakes fist*
'Tis the truth, the truth, me tells ya!
![=]](https://lvlt.forum.bioware.com/public/style_emoticons/default/sideways.png)
*Takes off tinfoil hat*
But seriously, I find it curious, I would have been interested in seeing someone else get together if Shep ignored them... :/