I think he'd be COMPLETELY different as a partner, than during the rest of his life's activities: He is someone who truly and completely lives for love and only for love - every action he does is for Lily, that's the only meaning any action could possibly have for him if it has any meaning at all. He was like that towards Lily both when she was alive and then when she was dead.

When it comes to love, he's not the Snape we know from classes and so on. He's completely romantic and tender towards the one he loves, as we can see it in his many Lily-related memories (even when they quarrel, you can feel this deep emotion he holds for her and this huge vulnerability he's got as the gifted and emotionally profound person he is, and this sadness because of love). (And it's also out of love that he becomes bitter and a bullying man, because love has such a strong impact on him. His every attitude comes from love. That's what makes him so different from other men and so special - cos normally you don't base all your actions and attitudes of your every day life on love, it doesn't normally determine every situation of your daily life. Normally you're able to act independently from love in your every day life, you're able to act normally, if you're a responsible adult. Snape, however, is downright capable of such insanity. He's completely gorgeous. (even if his behaviour is downright childish when he makes innocent people like Harry pay for his bitterness). But this childish, unconditioned love is what makes us love him so much I think, however unfair he was.

James, on the other hand, oh no, honestly, James, he's just so obsessed with himself, I don't think you could EVER get from him what Snape is capable of and willing to give someone he loves - that is to say: Snape just completely gives himself, he completely hands himself over to the one he loves (he even hands himself over to Lily via DD), his love is absolute.

James is just too busy with himself and the image he wants to create of himself, to be able to conentrate that intensively on a human being other than himself. He's compeltely loathable, what a nutter. He never had Snape's both intellectual and emotional profoundness, his capacity of very deep feelings - James is just so superficial, in my opinion.

Snape didn't give himself completely to Lily. He chose Voldemort over Lily.

Lily didn't choose the arrogant toerag James Potter over Snape. She chose the reformed character James who was kind and loyal and funny about a year after Snape had rejected her in favour of his death eater friends.

And as for Snape's superior intellect. This goes against what we're told of both James and Sirius being the best wizards in the school in their Hogwarts years.
And his "emotinal profoundness"? He was just obsessed with Lily. He never even bothered to get to know her properly, or else he would have known she wouldn't accept his obsession with the dark arts. He loved the idea of Lily. An imagined, idealised Lily. Not the real Lily.

Do you remember who said that? About James and Sirius being the best wizards at Hogwarts in their time? Because I tend to think that Snape was just as smart. He was inventing spells, for one thing, which we've never seen anybody else in the series do. And his Potions talent was just astounding.

J.K. said that the idea of Snape being in love with you was "horrible," and I tend to agree, despite my moniker.

For one thing, he'd probably be so possessive, you could say goodbye to all your friends...especially your male friends. I agree with The Sorting Hat -- that "greedy" look he gives Lily says it all: "creepy stalker!"

He'd probably also be the kind of boyfriend who calls you 20 times in a row, and leaves you 20 different voicemails if you don't pick up. Because if you don't pick up the phone, you must be cheating on him.

Then there's the fact that you can't take him anywhere. Without worrying about him terrorizing little children, or cursing any male waiter who came near you, I mean. ("I told you, we don't want any more bread! Sectumsempra!")

And can you imagine the head games involved in having a boyfriend who can read minds? ("You are thinking of a number between 1 and 10..." / "Stop it, Sev, I don't want to play anymore!" / "Six." / <sigh> "Yes, six.")

And then, if that weren't enough, there's the whole "let's-see-how-dirty-my-hair-can-get-before-somebody-actually-says-something-to-me-about-it" thing.

And yet...I haven't been able to stop thinking about him for 6 years straight...