When it is obvious that a relationship no longer holds happiness and all other positive things people crave, when it is obvious that the logical thing to do is to get out and seek happiness else where; both men and women have been known to remain attached to the toxicity of existing affairs simply because of the impressiveness of the sex they're getting from that bad boo.
Apparently, it's something about sex and its ability to make people take stupid steps and make moronic decisions.

Why s*x makes you misbehave

Canadian scientists carried out a research on this subject and published their findings in the Archives of Sexual Behavior in 2016.
In essence, they arrived at the conclusion that, the need for sex momentarily robs people of common sense because when you are horny, you get something known as ‘tunnel vision.’

In this context, tunnel vision makes you think of nothing else in that moment but how to release that sexual tension.
It won’t matter at that time that you’ll need to make a poor decision to get laid, all that’ll matter is to just get it one way or another - just ask Cardi B who returned to her cheating husband just to get some.
When you gotta go, you really gotta go.
Although this is not stated in the study, it is logical that when people have a sexual urge, they'd rather have it satisfied in the most mindblowing way possible. What this results in is a possible refusal to let go of a partner who satisfies their sexual needs fantastically only to deprive them of all other forms of happiness, excitement and satisfaction that relationships are known to provide.

Okafor's Law

One other reason why good sex may make people remain connected to people they should be severing ties with is the unfounded-by-empirical-evidence concept known as 'Okafor's Law.'
What Okafor's Law means in essence is that: if you sleep with someone once and you screw them properly, they somehow become too weak to say no to you from that moment onwards. Good sex is scarce
A lot of heterosexual women often complain about the cluelessness of men when it comes to satisfying women properly. This logically leads us to the inference that when a woman who has been with poor partners hits jackpot and gets with one who always leaves her breathless every time, letting go may be a little difficult, even when there's abundant evidence to the fact that he is less than an ideal partner overall.

In pursuit of blissful orgasms and happy relationships...

As ecstatic as it is to always reach mind blowing peaks of orgasmic pleasure with a partner, it should not cloud your mind or stop you from making right decisions.
What's left to say here is that whether or not the sex is good, any form of relationship that robs you of inner peace, happiness and renders you incapable of living your dreams and best life is not so good for you, and you need to let it go. Any partner that is only half-good for you is not good enough.
There's always the fear of the unknown, of course, but if you do not let this toxic one go, how would you know about the better one waiting out there for you?
There's a better man, better woman and better orgasms waiting out there for you - go get 'em!