Jade is a woman in her mid to late 30s who is clearly a very experienced woman when it comes to sex. She's married, yes, but it's an unhappy marriage, and whilst M&J were also in a seemingly unhappy relationship, theirs is still a young relationship and Jade's is one that has a lot more committments and ties, like the fact that they have children. As far as we know, she isn't sleeping around with anyone else, even her husband, when she's trying to start something with the MC so there wouldn't be much chance of feeling the dreaded 3-letter word.
It does however seem rather hypocritical that people are willing to give a pass to a woman in an unhappy marriage trying to start a sexual affair with her student, but not to 2 young girls going through a rough patch in their relationship and seeking solace in the arms of the MC. Yes, the MC knows full well that Jade's married and she is totally honest about what she wants from him, but she's older and less likely to have the kind of doubts and uncertainties that Maya and Josy have, and therefore she just tells it like it is. M&J, on the other hand, are still two 18/19 year old girls who haven't developed that same level of life experience to know how to handle a situation like theirs the right way, and they shouldn't be judged so harshly because of that.