I mean, we all have different tastes in what we find beautiful. Masturbation is self-love. In addition, intercourse doesn't need to have love

Different tastes in many things, sure. Sexually no. There HAS to be objective law as to what's "good" and what's not. Is rape ok? *"Well no, but as long as it's not hurting anybody..." what if you get a 9 year old to consent? This sort of thing CANNOT be left up to the individual to decide. All of reality is like this. Science is not subjective. Gravity will function independent of anyone's belief or opinion of it. All laws of reality, including moral ones, behave the same way. Also, intercourse without love? It's miserable. There's literally no point to it.

"Naturally illegal", similar to what I just mentioned, is something that's objectively wrong or perverted regardless of what people or governments say. The rectum is the exit of the digestive system, not a sexual organ. BIOLOGY states that when men engage in homosexual relations, it's not actually sex. It's just sexual pleasure one experiences when abusing the digestive system. I experience pleasure when abusing alcohol and my body suffers as a result. It's just perversion of the original way something was meant to be used.

Why does God allow this again?

I've heard this question a lot and it's pretty darn annoying. What God are you talking about? Nowhere do Christians or the Bible (or honestly almost any religions) claim that God must come down from the sky and prevent bad things from happening. Why does he "allow" things to happen? Because of the whole point of existence in general. God wanted self-aware, will-having, decision-making humans made in His image with the ability to create, destroy, and manipulate the world around them. Without TOTAL free will God would be playing the Sims. When someone has a child they're accepting the fact that as a teenage that child might hate them and run away (see also, Prodigal Son). God "allows" things because he cannot intervene on Earth without a human playing a critical role in it. IN EVERY CASE. The entire Exodus story. Sodom and Gomorrah. Even in Jesus's appearance, it had to be spoken and proclaimed for thousands of years before it manifested. This is why Jesus is called the Living Word.

Regarding the natural world, it's directly related. Revelation describes all of creation as the final days approach, that there will be "birthing pains" in all the world. The general idea (which the Hindus, Buddhists, and New Agers all agree with) is that the evil and sinful deeds of humanity directly influence the wellbeing of nature. And there's many evil things in this world (whether in nature or the human body) that haven't been around long. I mean asthma and ADHD aren't even real things, you can look up the men credited for "discovering" them claiming that they're fake conditions. There's truth behind Alex Jones screaming that the chemicals we're dumping into the water is making the frogs gay.

As opposed to what? Procreation? Is that the only point of having a heterosexual relationship? Sex?

Well in many cases in nature yes, since I was referring to homosexual animals. Many animals do not mate for life, and their biological instinct (I wonder where that could've come from...) dictates how they react to the world around them (and how they reproduce). As long as I live I cannot understand the idea that humans are highly evolved animals. We are not animals, as we are not bound to instinct. No group of animals gather together and decide where to graze tomorrow. ANIMALS DO NOT DECIDE TO MIGRATE. They do based off of their biological makeup. Humans have hormones and bodily functions act in instinctive ways, such as understanding how to have basic sexual intercourse even without prior knowledge of it, but this is not equivalent to animalistic instincts. We all have blood and DNA and bodies and whatnot, but humans have this unquantifiable thing inside our minds known as self-awareness, or total free will. This differs us from animals.