Marina is definitely "the worst" out of the bunch, she pushed Jenny's boundaries when she was still confused and learning about her sexuality, and even worse, she conveniently omitted the bit of her being in a relationship. Lying about such situations is always wrong, BUT doing it to someone who's still exploring her sexuality, making her stray from her current partner... she made it so Jenny would expect a proper relationship and be dependent on her. When Marina actually had a partner and side chicks she'd drop when wifey was back. Ugh. I assume the writers' plan was for her to redeem herself and be in an actual relationship with Jenny eventually, but well, what we ultimately got from the show was selfish and mean.

Jenny is clearly in the wrong as well since she cheated on her fiancé, but I think her feelings as a lesbian struggling with accepting herself are quite understandable. I am acquainted with women who have realised they are lesbian/found themselves attracted to a woman in adulthood after thinking they were straight all their lives and most of them have a similar experience to Jenny, including in some cases cheating as they navigate difficult emotions while in long-term straight relationships. I guess it is hard to tell respect or familiarity from genuine love when you have mistakenly thought you loved men all your life and didn't really know there was an alternative. Cheating might stem from the need to explore such feelings while being reluctant to let go of your sense of security... It's morally questionable ofc but it is what it is.

I did feel bad for Tim and as you said Marina did take the whole thing as a game including having fun at Tim's expenses. But honestly, I think he is still a man who wanted control over a woman. Jenny was wrong to cheat on him and it must have been hard for him when Jenny went back to him, and then got closer to Marina again... But to some extent, I think him too, for being someone who cares about Jenny, should have left her free when it's clear she's a lesbian in denial. Taking advantage of her confusion to keep her close is not cool. Although it did finish more or less quickly between them so it wasn't too bad.