I suppose it depends on context.
Like if we're coasting along happily and all of a sudden my partner is like 'you've been behaving didderently lately, are you cheating?' I might lose my shit too xD like, where the fuck did that come from?
But if you said 'you've been texting someone late at night and you've been more distant with me lately and some other stuff and I feel like you're pulling away and can't help but wonder if there might be someone else...'
I'd be like 'ahhhh.. I see how that looks! I'm so sorry I've made you feel like that even be a possibility. Of course I'm not, I love you'.
Basically a person who loves you tries to understand you and put themselves in your shoes. They have the emotional intelligence to understand why you might be feeling the way you are. They will then explain these things if possible. Eg: 'my best friend is going through some things with his gf atm hense the late night texts' and attempt to tackle the other relationship issues with you moving forwards. Open, honest, working together.
BUT an abuser will jump straight to 'you're crazy!'. And not be intetested in the fact that you have perfectly rational reasons to feel the way you do. They'll make out it's all your problem, in your head. They may even turn it round in you, implying you are or have been unfaithful. Or bringing up times where you have apparently wronged them or made them feel bad or unloved.
Or, storm off.
(But just to note, again, disclaimer, if this accusation is really direct and appears to come from nowhere, many people would storm off - as its rather an insulting thing to ask. That's why context and phrasing is key).
When dating abusers, you feel like you're the only one who ever tries to understand them. That you're constantly trying to understand them better and - make your own opinions easy for them to understand. Constantly explaining yourself and trying to prove your love/loyalty/goodness/worth or whatever it is.
Be aware however that abusers may constantly accuse you of cheating (often because - they are).
And having to accuse anybody of cheating In a relationship, likely marks the beginning of the end anyway. Because there's no trust there for some reason.