Different Layers?
Now, some people may wonder what I mean by different layers. This has nothing to do with the act of sex. Instead, it is more about the intent or reason to have it. I don't know how many people are part of which category and this may be arbitrary. Sometimes multiple layers may be intended or maybe none of the ones listed in this post. I can say one thing at least: Sex is not all about pleasure and with that said, let's tackle the respective layer.
Pleasure
The first layer is something that might be very common especially in the younger generation. Of course, it exists. If it feels good then why wouldn't people do it? There are some consequences with this though. Sexual diseases, a chance for impregnation, especially if something goes wrong and something can always go wrong (Murphy's Law). This layer probably applies to everyone to a certain degree, however, I think it differs in strength from person to person, so that other layers are stronger shown. It's kinda like a policy or rules that are checked one after another if you see it more technical.
A person who has their focus on this layer probably doesn't care too much about who they're doing it with. This means that there's a high chance for these people to be pansexual or omnisexual. Whether they are or not also depends on social influences, at least as long homosexual is seen as a weird thing.
Love
Some people may see this as the opposite of the previous layer, there's no doubt that one of the reasons for sex is because you love someone. Whether or not love is some biological thing that drives us crazy for a person or makes us wanna sleep with them is something I'm not gonna debate here. The fact is that people who love each other have an interest in an intimate, romantic or affectionate sexual time. I would assume people in which this layer is strong have a great interest in doing it with their special one only. (heh, obviously) Also, this usually has not so much to do with having the looks. It's not about bodily features but more about the relationship. The only downside of this layer is, that the feeling of love may change over time. From experience, I would say love does not go away or decrease of disappearing, it instead kind of calms down so to say. Most couples are probably together as long the love is strong and once it starts to calm down they break up as they don't feel like they "love" each other anymore. Worst case scenario they break up after having a child already. The saying that people are together "because they're used to being together" may not necessarily be true. I think it is possible to still feel attracted or love a person without those tingly feelings in the tummy or being all into them. As I already mentioned, it's a different kind of love; More calm, less wild.
Reproduction
If you're talking about sex, you just can't leave out the biological reason behind it. Reproduction or giving birth to new life is a part of sex just like pleasure and love or intimacy. Additionally, I think there's no doubt that reproduction is a goal for some people in the world. Just imagine being able to teach and show your child the world. I can imagine that being a unique and awesome experience. Though I have heard of people not having any interest in having children, birthing them into the terrible world as is, people hating children and some being afraid of the pain or exhaustion from the first few years, the crying and aggressiveness or jerkiness of children and on, and on, and on. Some may even say no, due to the costs that are created. Whether or not this can be linked to an unconscious attempt of regulating overpopulation is another topic on its own.
Rituals
I already mentioned that I journey through the world of magick, it's interesting, gives a different perspective of the world and how things work and it's really interesting philosophically. Also, it tries to answer questions we can't answer yet. Anyways, from magick, I know that sexual intercourse can be used for rituals. Sexual magick is actually a thing that is used to channel the energies that exist during intercourse and use those to fulfill your goal or wish. This is done by two people of any sex who agree to pleasure each other while having the intent, goal or wish kept in mind. The climax is then used to release the intent, goal or wish into the unconscious. This is then followed by total silence and forgetting the act in an attempt to allow the energies to do their job in fulfilling the intent, goal or wish. At least that's how it can be described. What exactly happens is hard to tell, as there's not enough information on that. At least I don't know the details I never participated in or read into details on this topic.
That's It
As I already mentioned, these are the layers I know of, as humans can be infinitely complex there can be infinitely many layers and all of them can have different weights. Additionally, you can also see sex as a ritual to become more intimate with the person you love mixing the ritual and love layer with each other. Also, don't forget that these aren't all layers, these are the ones I would say I have noticed or that I can imagine existing. Nonetheless, it is important to analyze sex and sexuality scientifically and to ponder over it to address it in video games or other media. The more you know about it and which type of sexual behavior matches which person and vice versa the easier it is to create a befitting character or to fit the right sexual behavior to the right character. It's also an element that further completes the style of characters and type that you want to create for your game and it's also important especially in sex games that currently only focus on the pleasure layer. Even if those games try to make a romantic character I've seen it fail too often.
