It is weird how Ian and Jeremy usually have a very wholesome bro relationship when it comes to Alison, both very eager to share her, while Ian is extremely possessive of Lena and her going for both is a breach of trust that even she is aware of, hence she doesn't commit to Ian if she had fun with Jeremy since she know it will hurt him if she goes for his friend.
Its another sign how the story prioritizes the Ian/Lena love as the true one, while all other relationships are mostly variances and secondary, Ian could never love Alison or Louise as much as Lena, just how all of Lenas male options barely have any good chemistry beyond physical attraction (and then there is Stan). Holly is the exception and also a love partner pushed by the narrative.
If Ian and Lena are being 'experimental' in their relationship why would Ian remain possessive towards Lena, since that is a contradictory position to the whole concept of being 'experimental'? That idea just doesn't make any sense, particularly if Ian has already had a threesome with Lena involving Louise, and knows full well that Lena has a similar fantasy involving two guys. Him refusing to countenance such a scenario, would be counter-productive, since the entire basis of this relationship is predicated on the two of them 'experimenting' with other people.
For me, Lena will always struggle in a conventional relationship, because temptation will always be there for her, simply because of the nature of the ways she has to earn a living. Whether she ends up as a rock star, a model, a dancer or even a pornstar, inevitably she'll end up in situations, where other men and women will desire her. And since Eva herself has made Lena's lifestyle and supporting personality this way, it seems totally contradictory that she would have Ian and Lena not move towards a fully open relationship, if they're 'experimenting'.
We all know why Ian has his doubts and fears, but surely he's intelligent enough to realise, that if he's going to date a girl like Lena, and have a successful relationship with her, then he needs to move beyond these fears and become more confident and open than he was, when he was dating Gillian. Because that attitude just wont cut it anymore, if he wants Lena to stick around. Otherwise he'll become increasingly paranoid, everytime his girlfriend inevitably is pursued by one of her multitude of admirers.
And if Ian is uncomfortable with such a scenario, why would he ever consider having a relationship with a girl like Lena, particularly when there are other, 'safer' options? The whole point of choosing someone like Lena, is you know it's never going to be dull, and probably somewhat of a rollercoaster, and if what you want is 'conventional', only a complete moron would go for that option.
If Ian wants a comfortable, conventional relationship, he should choose Holly or maybe Alison. Both of those girls are crazy about him, so he can't really go wrong. Lena is all about having an exciting, unconventional life, that will continuously push Ian to be more than he is.