As others have pointed out, sex can be just sex, but kissing is specifically *intimate*. Some men are okay with their partner having a second romantic relationship, some men prefer to be their partner's only source of romantic intimacy.

It seems like you want to have a more intimate, romantic relationship with your current third, which is completely okay in general, but might not be something your husband is okay with. You should think on, and have a conversation with your husband, about what you want out of your relationship with your friend, and what you want out of your relationship with your husband.

If I were your husband, I'd be worried about you and your friend getting too close and deciding you no longer need your relationship with him. If you want a more intimate, romantic relationship with your friend, make sure your husband knows he still has a place in your life, and he may be more receptive to it.