A whole lot of assumption here that no one, ever, can possibly have a brief fling without then self-combusting with guilt and shame, maybe even popping a scarlet letter on your front or being pelted with fruit by a baying crowd. Honestly.
Of course it is quite possible, more than possible and indeed likely, given you've thought about it and seem quite clear what the scenario is, that you could have a single merry evening with no consequences. Neither of you are likely to tell, don't fall for any temptation to start confessing all over the show - no, it won't make you or your partner feel better - and just get on with things.
Not everyone is going to be consumed with shame, 'feel used' or 'dirty' or other quite strange comments about how women are expected to feel after sex. If you think sex is dirty and shameful chances are you're not really up for affairs anyway.
"Because you become a liar" - oh, right, like no one in this thread has lies or secrets, ever.
The LTB crowd will all demand that any marriage, no matter how safe or stable, is worth tossing aside at a moment's notice, or that spouses can all be trained to sit and 'talk' through their feelings. Real life doesn't work that way.
So no, it is not naive or a foregone conclusion you'll feel bad about this. You have your own reasons for wanting it - DH no longer interested in you, maybe a bully, dull, probably doesn't talk to you any more and spends his weekend arranging his spanner collection, yells in your face if you try and talk to him about anything, controls the house and money so you can't leave. Fuck it. He's driven you to this, so what's to lose?
I mean, if none of that really applies and you actually can leave, maybe consider your marriage is pretty dead and it might be time to move on.