Cheating, as in relationships, is not an upstanding thing to do. It is exemplary of an immature, confused, untrustworthy, undependable character. Let me be clear: I don't think it's wrong to fuck around. But it's important to be open about it. Relationships and sex are a big deal to people. If you're going to be doing that with more than one person, make sure that everyone involved is aware, and understands.
Now, that being said - there are those who hate cheaters. I must say I dislike that more than cheating. When it comes to cheating, the well-adjusted person is one who neither cheats, nor hates with a passion. But if I had to choose between a cheater, and someone really sour about it...
The cheater, you see, is only selfish in their seeking of pleasure. But the hater is ignorant and bitter; coercive and entitled in imposing their views. These views tend to be naively monogamous, as if to imply: My expectations should be met. This should just work. I shouldn't have to try to find the right person for it.
Cheating happens within a relationship. If you get cheated on, then you, also, fucked up. You accepted a task: to pick a person, and create a mutually functional relationship with them. You could choose anyone in the world. This is the person you chose; this is the relationship you built.
The pain of being cheated on is emotional. It comes from within, from a clash between expectations and reality. If you didn't take the relationship seriously, there is no pain in the first place. But if you did, then there can be great pain. Those who would rather blame their partner than deal with their emotions try to rationalize reasons to see cheating as an objective wrong, rather than a trigger for their pain.
One way that people try to make cheating an objective wrong is by arguing that cheaters expose their partners to health risks. Yes, technically. But I disagree with a majority of what this is meant to imply.
If you're alive, chances are you've had the flu a number of times, and you're not holding grudges against people about it. You've probably infected people with your flu, and you aren't expecting to be blamed. It's only in the sexual realm that we have this insane expectation that an infection is living proof of shameful sin, instead of something that just happens to people, and that we're trying to solve with medicine.
It's also not your partner's job to protect you. It's your job to protect you. Safety is not something you check off and then forget about. It's something to keep in mind, even with your long-term partner.
Finally, you chose your partner. By choosing your partner, you're choosing all the baggage that comes with them, STDs or not. If you're a poor judge of character; if you create a relationship in which someone cannot be honest, is being coerced, where you don't communicate; if your partner keeps secrets from you, and you don't detect this; all of that is your blindness. All of that is not only on them, but also on you.
A hater of cheating wants to have their cake, and eat it too. They want to have sex, but not incur any of the risks associated with bodily activity. They don't want to take responsibility for choosing a partner. They make themselves vulnerable while building a relationship whose parameters are downright adversarial. Then, in order to feel safer in this choice, they'll consider themselves owner of their partner's genitals.
The cheater seeks only pleasure. The cheater hater seeks to clutch love in their fist. They build relationships on top of taboos that discourage communication. They seek to coerce, to ignore; to make their partner into something they're not. Then, they blame their partner for dishonesty and betrayal.
Next similar post: The ethics of non-consensual monogamy: coercion and dead bedrooms