A deep state of relaxation is one of the common consequences of being hypnotised. Usually the process of hypnotising someone involves telling them to relax and---surprise, surprise---they end up very relaxed.
In terms of being a fun sexual partner, being totally relaxed doesn't help; but because they are both physically and mentally relaxed, many of the obstacles to strong sexual arousal are gone. These obstacles could be distractions or worries from work, nervousness, performance anxiety, and so on. As they go deeper into a trance these all become less of an issue and, eventually, can become completely irrelevant.
At deeper levels of trance, your partner can be physically active and an enthusiastic participant in the sexual things you do together. The mental relaxation can remain though, and they are then able to focus just on the sex without distraction.