Bella was madly, romantically in love with Voldemort, and obsessed with him. Alan Rickman knew early on that Snape had been in love with Lily, so he...
"Bella was madly, romantically in love with Voldemort."
- Jo's appearance on PotterCast (Dec. 24, 2007)
Bella was in her 40s [bft] but voldie was in his 60s[National Press Club, 1999]
Isabel: Did bellatrix ever love her husband, or did she have love only for voldemort
J.K. Rowling: She took a pureblood husband, because that was what was expected of her, but her true love was always Voldemort.
[Bloomsbury Live Chat (Bloomsbury.com), July 30, 2007]
JK has said that voldemort liked her, but obviously never loved her (no ref?)
we see near the end of Deathly Hallows, he screams whenever Bellatrix dies, and heads straight for Molly.
She spoke to him as if a lover. "My Lord." [Deathly Hallows US]She is described as regarding him with "worshipful fascination" and speaking to him as if to a lover
Voldemort/Riddle has never cared for or loved anyone. "He couldn't possibly be what he is." [Read the exact quote from the Edinburgh Book Festival, 2004]
Has Voldemort or Tom Riddle ever cared for or loved anyone?
Now, that’s a cracking question to end with—very good. No, never. [Laughter.] If he had, he couldn’t possibly be what he is. You will find out a lot more about that. It is a good question, because it leads us rather neatly to Half Blood Prince, although I repeat for the millionth time that Voldemort is not the half blood prince, which is what a lot of people thought. He is definitely, definitely not.
DH(P.16, UK Hardcover): Many(Death eaters) would call themselves his(LV) most loyal, claiming they understand him, and that he confides in them...
The fact that Bella and Rodolphus never had any children, but she would have giving them all up for the dark lord. bella to Narcissa in HBP, "If I had sons, I would be glad to give them up to the service of the Dark Lord!"
" ... where Narcissa sat rigid and impassive, Bellatrix leaned towards Voldemort, for mere words could not demonstrate her longing for closeness."
[Harry Potter and the Deathly Hallows (The Dark Lord Ascending)]
While Snape and Lily had a caring relationship, that could have turned into a marriage if Snape wouldn't have chosen the dark path as Jo said, there never would have been any romantic relationship between Bella-V. there was no anchor to it. For me V. represented more the "daddy" she never had and tries to please him and to have him on her side, more of an incestous relationship.
Bellatrix: "My Lord, it is an honour to have you here, in our family’s house. There can be no higher pleasure."
Lord Voldemort: "No higher pleasure... That means a great deal, Bellatrix, from you."
Bellatrix: "My Lord knows I speak nothing but the truth!"
[Bellatrix's delight at Voldemort staying at Malfoy Manor DH]
"Potter, you cannot win against me! I was and am the Dark Lord's most loyal servant, I learned the Dark Arts from him, and I know spells of such power that you, pathetic little boy, can never hope to compete..." [OOTP]