Williams presents human sexuality as something that can, under certain circumstances, lead to one's destruction. That's precisely what happens to Blanche DuBois, whose own sexuality is a major part of who and what she is.

Blanche has the great misfortune to live at a time when female sexuality was supposed to remain firmly within the confines of heterosexual marriage, and even then, it was purely for the purposes of procreation. Blanche's sister, Stella, is also highly sexualized, but the big difference is that she's managed to channel her sexuality into her marriage with Stanley. Stella's pregnancy with Stanley's child is the outward manifestation of what society regards as acceptable in relation to female sexuality.

A double standard applies, however, in that similar strictures on male sexuality don't exist. Stanley doesn't just cheat on Stella by having sex with Blanche; he actually breaks the law by committing rape. Yet according to the warped values of traditional society Blanche is the one who needs to be institutionalized whereas Stanley can look forward to a long, happy marriage with Stella.

On this reading, it's not so much that human sexuality leads to destruction per se. What matters are the prevailing conditions in society. That being the case, Blanche ultimately comes to grief not through her sexuality but through the double standards of a society that cannot handle the expression of her sexuality.

In A Streetcar Named Desire, Tennessee Williams depicts sexuality as an inescapable and destructive force which benefits only heterosexual males. Each of the main characters succumbs to sexual desire in a way that suggests that humans have little to no control over their sexual impulses. When Blanche states, "Death—the opposite is desire," she establishes the proposition that as long as one is alive, he or she will act on desire. Blanche herself indulged in sexual promiscuity as a way to escape the atmosphere of death at Belle Reve created by the repeated succession to the graveyard for funerals of her dying relatives. All the characters in the play seem to give in to their sexual urges without putting up much fight. Thus Williams seems to indicate that sexuality is a normal part of life.

Although sexuality is normal, it is also destructive according to this play. Blanche's husband, Alan, kills himself because of his homosexuality. Blanche destroys her reputation by being promiscuous, and she gets fired from her job as a schoolteacher for engaging in sexual activity with one of her students. Stanley destroys Blanche's sanity—what is left of it—by acting on his sexual impulses and raping her. Mitch would also possibly have tried to force himself on Blanche in the scene when he confronts her with what he knows of her past. At least he shatters her hopes of marriage and protection. Stella's sexual desire led her to marry a boorish, selfish person and keeps her under the thumb of an uncultured, poker-playing, violent man. She is content to live a lower-class lifestyle as long as she can have those "colored lights" flashing. Stella's account of her wedding night when Stanley smashed all the light bulbs in the flat with a slipper is a metaphor for the destructiveness sexuality. 

Despite the destruction it causes, sexuality can benefit some—if they are male and heterosexual. Thus Stanley faces no repercussions from forcefully using Blanche for his sexual desires. Mitch also presumably has no qualms about sex outside of marriage for himself, even as he condemns Blanche for having engaged in it. Alan, the homosexual, and Blanche and Stella, the women, are left to suffer the consequences of their sexual desires while the men, Stanley—and to a lesser degree, Mitch—can go on with their lives without feeling any negative effects. 

From the experiences Williams portrays in the play, he seems to present human sexuality as inescapable, destructive, and beneficial only to male heterosexuals.

Tennessee Williams explores the disastrous results of unrestrained promiscuity throughout his play A Streetcar Named Desire. At the beginning of the play, Blanche says to Eunice that she took a streetcar named Desire to another car named Cemeteries, then was dropped off at Elysian Fields. This journey allegorically represents Blanche's past, present, and future. Her unrestrained sexual desire led to her excommunication from Laurel's society, which emotionally ruined her, leading to her inevitable downfall in New Orleans. Blanche also mentions that the reason her family lost Belle Reve was because of her ancestors' “epic fornications." Additionally, Stanley's character epitomizes sexual desire and masculinity. Stanley is able to control Stella through their purely physical, debased relationship, which happens to excite and enchant her. He also ruins Blanche by raping her towards the end of the play. Overall, Tennessee Williams portrays human sexuality to be a powerful yet destructive force in the absence of self-control. 

Tennessee Williams, in his play A Streetcar Named Desire, alludes to human sexuality through the characterizations provided through the strong use of gender roles.

Stanley is characterized as the stereotypical strong male. His dominant male sexual nature is seen through his abuse of Stella and the rape of Blanche. Through both, Stanley is able to control his home. (He is able to manipulate Stella into doing as he wishes--by getting her to continually take him back. He is also able to rid himself of Blanche by raping her (which results in her complete mental breakdown).)

Blanche, on the other hand, is a very sexual creature as well. She has used sex in the past in order to survive (as seen through her history as a prostitute). Not only does her sexual past ooze into her present (through her seduction of both Stanley and Mitch), she believes herself to be far more sexually attractive than she really is (given her aging).

Essentially, Williams is stating that human sexuality can be seen as a tool of power. Both Stanley and Blanche use sex to gain the things they need (Stanley to control his home and Blanche to control her desire to stay young).