I think introducing Charlotte as 'bi curious' could be done using reflection about she and Denise when they were younger.
Kissing practice might be a bit of a porn trope for lesbian action but it would be a way to plant a curiosity that had not been really explored before the present.
Some reflection and 'remember when' between Charlotte and Denise over wine (overheard by the MC) could offer a choice for the MC to interrupt (stop that content) or listen (let it play out).
Denise is, IMO, the wildcard who can best make this happen - so we have to see how the relationship between the MC and Denise builds as well as what Denise sees/is interested in.
Cheers!!