Answer:
Boys/Men need to be mentally or physically aroused by being sexually stimulated by someone else during sexual intercourse, by having enjoyable sexual thoughts, or looking at any of various displays of nudity or sex. As the penis is held and rubbed up and down or thrust repeatedly into a sexual orifice of someone else, the mental and physically arousal work together to basically encourage the penis to ready sperm in its testicles (the two "balls" in the sac attached to the shaft of the penis) for ejaculation (getting it out of their body). The climax of the sexual arousal causes the release. Also, lubrication makes the sexual stimulation more intense and readies a release more quickly, especially if the physical stimulation causes even greater mental arousal.