Basically what Piff said, Astarion can make his decisions, before the ritual he says no, afterwards he gives it a try but wasn't ready for it. The only issue is, that you cannot show concern for him when things go wrong.
You also do not cheat on him in Act1 or early Act 2 respectively, because you are not in a relationship with him at that time, you are just a casual bit of fun with no strings attached. The conversation, I mentioned, is a very nice transition from the fake manipulative situation to something more real (which I found very rewarding). I wouldn't call it an encouragement to date multiple characters though, it's more a thing that comes up if you did. In Astarion's case - as mentioned in the other thread about dialogue options - it would have been nice to have dialogue to address how fake the moonlight date or the follow-up rendezvous are while they happen because I feel this would be important or at least an interesting conversation to have.
There also is no need for a special fidelity scene because fidelity is what most characters assume to be normal. For Astarion your commitment during Act1 only indicates that his plan works, there is no additional friction that would require him to think about those pesky feelings or open up about them. But as Piff said, there are already more than enough threads about Astarion specifically.
