Let's get theoretical, Veeky Forums.
Think of your favorite non-evil paladin/cleric character you have, if you have one.
Got one? Okay, so your holy man and his party is in between adventures and chose to spend some time in a sizeable city or town, filled with all kinds of different races.
So, you're going through your daily routine when you encounter a girl; she's reasonably attractive, of age, and is your prefered race. Now, for the sake of brevity I'll just ssy shenanigans happen and poof, you're beginning to fall for her. You even think you might be in love.
You both spend time together, and after a week or so the both of you sit around in a sparsely populated area. Then, out of habit or just because you can, you cast Detect Evil. Everything checks out normal...
...except for the girl, who pings as a feind. Uh oh. One Dispel Magic later, and her glamor falls. Boom, this grill is a succubus. Zone of Truth time.
She says she doesn't intend to suck anybody dry, she just wants to live peacefully and she really, really likes you, wink nudge.
So, assuming the DM isn't trying to bait you into falling, getting smote by your god, or magical realm, do you continue with the relationship knowing full well the ramifications of it? She's a succubus, so any hanky panky will likely result in a dead holy man, and even kissing her takes your breath away in the litteral sense.