Intimacy and Kissing
That's a question that should be asked of your partner. Kissing is a very intimate act. It creates too great a sense of closeness, a commitment that they do not want to make. Prostitutes often do not kiss their clients for this reason. They simply avoid it. Kissing is the first sensual act we learn. We kiss our parents and our children. So can become the most intimate sensual act, whereas during sexual intercourse we can detach ourselves from the experience. It becomes simply a physical act rather than an act of connection to our partner.
You mentioned all of the adventurous things that your partner is willing to participate in. But you do not say anything about intimacy. Activities that are purely sexual in nature are not necessarily intimate. They are playful. Kissing makes the event more intimate. Furthermore, in order for a couple to enjoy kissing, they must practice. However, some people have difficulty telling their partner that they do not like the way the partner kisses...or that their breath smells.