Do humans and demons have romantic relationships with each other in your setting...

Should it end in more or less pain and misery than attempts at romantic relationships between humans and angels?

Angels will do things to your butt due to all the repressed sexual urges while demons are actually super cuddly.

More pain. In order to be with the demon forever, you have to go to Hell. In order to be with the angel forever, you have to go to Heaven. The latter is more difficult, but it also is far less painful.

Angles are dicks only because they're lawful stupid

they do fall in love with humans/Elves in my world, but honestly it might be better for a person to hook up with a demon in my world.
Angels will not be nice at all and be super controlling of their spouse if they decide to be physically with them; probably condemn them to hell anyways due to adultery or messing with the order just to save face or something.

I can imagine doomguy wasting the demon and just slowly shaking his head at the human.

Demons are fallen angels, they do not hate the mortal races so much as they envy their freedom. So they sought ways to create conditions where said mortals humiliate themselves, make beasts of themselves; in a vain effort to show to a Creator no longer caring or even able to change things they were the superior choice.

As you gaze into the Abyss, the Abyss gazes into you and thus they made monsters of themselves in the process; tempters obsessed over mortal races debasing themselves in order to debase them. Many naive mortals mistake this for love. A rare few naive demons even mistake what they feel for love and try to ascend above their nature. But these unions, even when mutually desired, rarely end well...

Wouldn't he kill the human as well?

Or not. Doomguy's backstory makes him seem like a pretty good guy, not a psycho. He's on Mars because he refused to shoot civilians, and he fights demons because they are invading. He's a soldier, they are enemy combatants. So I imagine it'd come down to whether the demonic partner counts as a civilian or not.

Campaign idea: a demon wants its superior assassinated, takes humanoid form, seduces one of the players.
Or: a devil lets a mortal enslave itself, because devils are always part of a hierarchy and a mortal master is easy to manipulate, especially with sex.
All fiends are attracted to strength, demons to brute combat prowess and devils to cunning. They try to make mortals fall simply because those still resisting them are irresistible to them. But should the mortal fall, their soul becomes far less of the price once in the Lower Planes and is eventually abandoned, since the light which made it precious is gone - in essence you died long before your body did - and eventually becoming a fiend itself, drawn to that which they have lost and beginning the cycle anew. So the only way to have a happy ending is to stay adamant, tempt the fiend to repent and drag them to the Upper Planes with you.
Consequently, a romantic relationship with a fiend is a constant battle of wills and wits over your very soul, and theirs as well. Simply having one won't damn you, just make your damnation pretty damn likely.
Also, the reason behind reason paladins exist is to provide irresistible and incorruptible waifubandos to fiends and win the conflict between good and evil through attrition.

Humans and Demons romance eh? Perhaps this setting could help you satisfy such fetish/tastes/cravings:

>>mgewiki.com/w/Monster_Girl_Encyclopedia_World_Guide_I:_Fallen_Maidens

>mgewiki.com/w/Monster_Girl_Encyclopedia_World_Guide_II

>>mgewiki.com/w/Monster_Girl_Encyclopedia_World_Guide_I:_Fallen_Maidens

>mgewiki.com/w/Monster_Girl_Encyclopedia_World_Guide_II

no, user

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