Yes, that is certainly one aspect of it. Perhaps some guys get aroused by seeing a big penis because they see so many in porn and, even though they were originally searching for images of women, they now associate seeing a big penis with the arousal they get from porn. I think this might be a good time for me to put forward my treatise on human mating: In general, guys are genetically programmed to mate with as many women as possible, yes? Regardless of how we might feel about it, it's simple biology and evolution. More mating means more offspring and they are therefore more reproductively successful. Their DNA has been passed on to the next generation. However, men are also programmed to fall in love and stay with women. Most men have fallen in love at some time. This might also yield reproductive success. By staying with one woman, they might have a better chance of successfully raising one or more children to adulthood. Some men choose the strategy of sleeping with as many women as possible. Others choose monogamy. And, some might choose a mixed strategy . . . but we all feel at least some pull in both directions. Culture, society and peer pressure obviously play a HUGE role in the choices that we make. And, so does our desirability to the opposite sex. And, so does our ability to physically overpower potential rivals. A really strong man might go after many women and worry little about upsetting their boyfriends. And, a really good looking man might try to sleep with many women, but might be more careful about getting caught. A guy who does not have much luck with women might put forward his fidelity as an attribute to try to secure a woman who will mate with him. And, for some men, even raising one or two children of another man might be worth it if he can ensure that his wife will bear him at least one offspring. Now, women are also wired for promiscuity and monogamy. It's definitely more complicated for them, especially as they are going to be raising their offspring and will want to have a man to help them, especially during the first few years. Monogamy makes a lot of sense for most women (as they will successfully rear offspring), while unbridled promiscuity does not (as they will have offspring, but will have a hard time finding support). The best strategy for most women is a mixed strategy. Women are most attracted to monogamous type males (provider, husband material, beta male, nice guy, etc.) during most phases of their menstrual cycle, but they are most attracted to the promiscuous type males (lover, stud, alpha male, bad boy, etc.) when they are ovulating. Despite strong cultural pressure to be monogamous, studies have found that 40% of women have at least one sexual liaison outside of marriage and that 10% of children born are not the genetic offspring of the man listed on their birth certificate. Typically a woman who does end up cuckolding her husband will make love to him on a regular basis and then have a "one-night stand" right as she's ovulating and she'll often make love to her husband within 24 hours of the encounter. The routine sex leaves the husband feeling secure that his wife is faithful. Furthermore, the woman often feels assured that the child is of her husband when she conceives because the affair "only happened that one time". If a woman does manage to cuckold her husband, she will have greater reproductive success. The faithful cuckold will help ensure that all of her offspring are raised to adulthood and yet her genes are more likely to succeed because she benefits from having diversity (multiple fathers). One might take it a step further to suggest that her offspring will do better because they will benefit from the superior genetics of her lover. Why do women moan when they have sex? To let other males know that they are ovulating and looking to reproduce. Why do men tend to stay quiet? Because they don't want other men to mate with their partner. Why is there a head on the shaft of the penis? It is to function like a plunger . . . removing semen from the walls of the vagina which was deposited by another man. And, why is a bigger penis superior to a small one? A longer penis will deposit semen closer to the cervix and out of reach of a small penis . . . all while more effectively removing the semen from the smaller penis. This hits the nail on the head!! Typically we see each other in a flaccid state . . . and the only way for a small guy to "measure up" to a larger guy is to get an erection. Similarly, if a small guy suspects that his girlfriend or wife has been unfaithful, it's a primal response to get aroused. If he can make love with his girlfriend right away, this would enable him to at least deposit some of his own semen and possibly remove some of his competitor. And, being small, he might get extremely aroused because he needs to be as erect as possible in order to increase his rather slim odds of success. Agreed. It's absolutely humiliating to get an erection when seeing a guy with a bigger dick . . . especially if there are women around!! One would think that my biggest fear when skinny dipping or going to a nude beach would be that everyone would see how small I am. But, to be completely honest here, my biggest fear is getting an erection! Then people will wonder why I'm getting an erection. They'll think I'm a pervert. And, they will see how small it really is. Once you have an erection, you can't hide behind excuses like "Oh, it's just really cold" or "I'm a grower, not a shower".