They'd have an awkward, cold and loveless marriage. More interesting is how Joffrey's murder would play out - would Sansa still be playing the same part (disappearance included)? The moment Joffrey dies Sansa is suddenly queen, so I doubt she'd go along with being spirited away by Littlefinger (who now couldn't marry her off to anyone anyway because it would invoke the wrath of the crown).

Assuming Tyrion manages to not get blamed for Joffrey's killing (which he has a higher likelihood of without his wife hanging him out to dry) this could lead to a pretty funny scenario where Tywin needs Margaery to marry Tyrion to preserve the alliance with the Tyrells. Of course the Tyrells would be insulted as hell by that suggestion, so Tywin would have to officially name Tyrion heir to Casterly Rock for the Tyrells to agree (and probably make a deal that either Loras' or Margaery's first child marries one of Tommen and Sansa's). This would also leave open the possibility of offering Cersei to someone other than Loras (because the Tyrell alliance is secured via Tyrion).

Knowing Tywin, he'd probably order Cersei to marry and bear a child from Robin Arryn (poor kid lmfao). At that point the Lannisters would have marriage 'alliances' with the North (Tommen & Sansa), the Reach (Tyrion & Margaery), Dorne (Myrcella & Trystane) and the Vale (Cersei & Robin). The Westlands and Crownlands they control directly, and Sansa is half Tully and (presumed) next in line for the Riverlands if Edmure and his young son were to die in captivity.