Many animal species have a season in which they become sexually active: their behavior, and even appearance, will change in preparation for sexual activities. However, a few animal species remain sexually active after maturity. And of these some even have distinct sexual appearances before maturity. Humans are sexually distinct upon birth, and even before maturity have different behaviors. Human sexual form and behavior is divided among male, female and transsexual forms exhibiting heterosexual, homosexual, bisexual and aesexual behaviors in immaturity, maturity and senility: a total of 36 distinct sexualities. The sexuality of a person does not remain the same during their lives, but is subject to change.
Yet humans are different than all other animals in that they undertake sexual behaviors through conscious intention, not merely through instinct. This consciousness is the origin of the Dharma of Kama: the volitional behavior results in Karma. Some behaviors are required by duty, others are not required by duty, others are prohibited by duty and still others are not prohibited by duty.
Not only with sexual behavior, but all pleasure: humanity experiences pleasure consciously: some pleasures are required by duty, others not required, others prohibited, and others not prohibited by duty.
The Dharma is often mis-interpreted as being toward abstinence and asexuality. The grounds for this are of diverse and dubious reasoning. It is argued that by abstaining from pleasure or practicing asexuality a person performs a sacrifice of pleasure or sexuality: as a person would lend a copper coin and obtain a gold coin tomorrow, it is argued that this sacrifice earns considerable merit. But this is an error: sacrifice and prayer alone do not secure a goal. How many defeated armies have prayed for the destruction of their enemies? How many crops have failed for lack of rain - despite the devout sacrifices and prayers of the farmers? No crop at all will be harvested if a seed is not planted: do not pray or sacrifice for what is desired. Rather, it is wiser to understand the causes that condition its accomplishment, and effect your satisfaction. The moon does not work for the betterment of the shore by pulling tides in and out: it the nature of the moon to do so, the moon does not work. It is human nature to experience love, to experience pleasure, to behave sexually. It is important to undertake this Karma Yoga in accordance with the Dharma.
There are also those who would argue against sexuality and pleasure, saying that its impermanent nature is reason enough to dissuade its pursuit. However, the impermanent nature of pain is no argument against avoiding it: we do not rest our hand on a a hot stove just because it will one day heal, and stop hurting. The pursuit of pleasure and the avoidance of pain are both ignoble, and fruitless pursuits - as much as the avoidance of pleasure and the pursuit of pain. The exertion required to gain, to enjoy, to guard against loss, to succeed, to ward off defeat - this exertion is properly understood as Karma Yoga. Even if destiny certainly, eventually, holds defeat and loss a human must still strive, for it is human nature. It is the act of striving which is the origin of contentment, and happiness. Would a farmer not plant next year's crop just because deer might eat it?
There are those who would abstain even from pleasurable pictures to decorate their walls, thinking that pleasure brings distress: when, indeed, is pleasure not followed by pain? Indeed, the pursuit of pleasure leads a person to commit unrighteous deeds, regardless of the future and consequences of their actions. Ignobility and infamy result from the pursuit of pleasure. However, the same is true for those who seek pain, or to avoid pleasure. Pleasures are necessary for existence: food is pleasurable, as is water, as is bathing - all pleasures are the result of striving, and the results of Dharma and Artha: the striving must be governed by caution and moderation. Would you withhold food from a begging Sannyasi because the food gives pleasure?
Practice Dharma, Artha and Kama, and enjoy benefit now - and later. Perform Kama without fear of danger to your welfare. With the following exception: if Kama is performed without Artha or Dhamra, it should not be performed.