How do the themes of sex and maturity relate to Death of a Salesman?
Consider the way in which Happy "ruins" the fiances of the bosses for whom he works. Because he's jealous of their success and his lack of relative stature within the company, he uses his skill for luring women into his bed to get even with them. This alone is enough to demonstrate a clear theme of sexual immaturity, as he uses intimacy and physical love as a means of revenge, rather than showing true care and affection.
Secondly, Willy is also an example of this theme. When he has an affair with the Woman in Boston, he is using sex as a means for feeling important. Willy has never been an overly successful salesman, the readers are safe to assume. However, it is correct to assume that he did have some sort of charisma, as he attracted a woman who was willing to have an extramarital affair with him. However, such quotes as "bottoms up" and the like show that for him, it's a power struggle, similar to Happy's use of women, in that he wishes to perpetuate a stereotype of being a womanizer and a socialite.