Green’s Dictionary of Slang

1. (US prison) an act of intercourse; to have intercourse.

1940

D. Clemmer Prison Community 330/1: beans, n. A copulation. v.i. To copulate.

2. (US) semen.

1950

J.E. Schmidt Lex. Sex 17/1: beans. The ejaculated seminal lava [Simes:DLSS].