Wallaby, you read threads on here, yes? I'm sure you've heard the pain of Mumsnetters realising their husbands are having emotional affairs. They suffer the particularly sharp loneliness of living with, and sleeping with, a partner whose thoughts and feelings are elsewhere. They're confused because, when they beg their partner to tell them what's wrong - does he want to end the marriage? - he says No, I love you and the kids and this is my home. I will never leave you. While saying this, he's still checking out emotionally every so often, stealing time and energy from the family he swears he loves. No wonder she's confused. He gets ratty with her: criticisms that make no sense except as comparisons with some other woman. Perhaps he's distancing from the kids, blowing hot and cold; leaving them needy.
She pours it all out on here, and we say Yes, it's hard to face but he is having an emotional affair. It's like a crush, he thinks she understands him like no other. You must feel bereft. I feel confused, she replies, he swears he loves me! We still have sex! But he's not altogether 'here' :(
We advise her, don't we, to take some time away from him. We say it will help to clear her mind.
It's likely he will tell his other woman that his wife's taking a "secret holiday" preparatory to divorce. He would say that, wouldn't he? It feels good, for him, to have OW's sympathy poured over him like honey. It's far easier than making an effort to engage in the talks his wife has anxiously requested. OW is flattering, undemanding, easy - and he is lazy.
That's you, that is: flattering, undemanding and easy. His wife's going away for some head space, because her husband is selfish and emotionally lazy. She's stopped being "easy", so he turns to you.
If you're okay with being the easy option, go for it. Frankly I think you may as well have sex with him; it won't make much difference to the other relationship now. And you'd better be prepared to be undemanding, easy, and so on for the duration of your time with him. Because you know what he's like, don't you, now?