why would that be too much? it is logical for the story - she is in a fact (filthy route or not) very prude & loyal woman/wife. that could be the trigger when she decide to have sex too
If Liam becomes playable character it would overcomplicate the story in many ways (as this will require adding more scenes on some days). It will require to Liam also have some sort of Filth points or just tie Sofia's possible actions with Liam's choices.L&P was planning to do liam a playable character it was to much.
none of the character of liam job are in the official charapter guide.
On the other hand, it would make sense if somehow Sofia discovers that Liam is more friendly with other girls then he should be she can start to questioning herself "If my hubby is trying to hook up someone else why I can't do the same". Of course it can mean that Liam can only flirt with some girls and on some point Sofia is just trying to find excuses for her to have sex with another man.