I know I am not the only one who thinks that firstly Sophia needs a tipping point event to actively cheat on Liam, and secondly that we have not such an event, yet.
I think
Apollodore might be on the right lines thinking that could be tied in with Ctahy & Andy, either this meeting or the next one. If Liam shows that he is too eager to have sex with Cathy (to help them have a baby if Andy is in fact infertile), then even though it is not Liam cheating on Sophia if she agrees to it, BUT it could allow Sophia to think "Well if it is ok for Liam to have sex with another....."
Even if fertility is nt used as such a plot point, could still get to a similar scenario if Liam pays too much attention to Cathy or he is ok with Sophia and Andy flirting (personally I don't think the latter as Liam has previous for showing jealousy re Sophia).