Stanley is sexually attracted to Blanche, but it is an unhealthy lust fueled by insecurity, toxic masculinity, and an aggressive desire to dominate.
The play emphasizes that Stanley is a sexual being, aggressively earthy in contrast to Blanche's pose of ethereal innocence. His relationship with Stella is held together by sex, and Stanley tends to look at women as sex objects. He "grins through the curtains" that provide the thin separation between his and Stella's bedroom and where Blanche sleeps in a leering way: the word "grin" is frequently applied to Stanley and seems to be a stand-in for leer—it is a threatening and grotesque facial gesture. Before raping Blanche, Stanley says, "We've had this date with each other from the beginning," suggesting that sexual possession of her has been on his mind since she arrived.
Stanley feels insecure around both Stella and Blanche because they represent a higher social class. When Blanche insults him, he tells her, "I am the king around here," and this need to assert his dominance is a sign of his insecurity. This—and the larger culture—leads Stanley into a toxic masculinity in which it is difficult to untangle sexual attraction from aggression and power plays. It leads not to any kind of lovemaking with his sister-in-law but to a violent sexual assault that destroys Blanche.