I agree with you. He’s definitely gay.
Just to point out that strictly speaking, a lavender marriage is when a lesbian and a gay man enter into a marriage for social convenience and often so they can have children. Julie Andrews and her husband Blake Edwards were supposedly an example of this arrangement. Also back in the day Vita Sackville West and Harold Nicholson married for social convenience and to produce heirs. Vita known best for her affair with Virginia Woolf.
I suspect that Steph married PS simply because she loved him, and he obviously had very strong feelings for her and thought maybe the gay would go away if he married her and had a ‘normal‘ family life. He wouldn’t be the first gay man to do that.
I doubt she knew from the off that he was gay. Possibly he told her he was bisexual but no more than that. Maybe he believed that himself? I’m guessing he maybe stayed faithful to her for years but eventually realised he was actually gay and told her at some point once his kids were grown up, saying he didn’t want to act on it, but needed to be honest with her.
I suspect his daughters weren’t told until relatively recently. Their mother assured them that she was OK with it and that they still planned to stay together at least for now. The MM thing meant he had to come out publically.
That’s my reading of it anyway.