At first it was just a comfort thing. They were so lost and lonely during those cruel months at Hogwarts, and found solace and protection in each other's arms. The world around them was falling apart, bit by painful bit. In the midst of destruction and pain, they found their safety, nestled between Neville Longbottom's bed sheets- only Seamus shared the room with them now, and he wouldn't tell. He was too busy tending his own wounds and dreaming of someone else.
It was a comfort thing, and it was a healing thing too. They fed off each other's strength and bravery and desperately tried leading the pitiful rebellion they had formed against the Carrows and Snape.
Maybe that's all it ever was to Ginny – something to cut the bitter taste of loneliness and take away some of the fear and uncertainty. Harry wasn't around, and Neville always knew that if it hadn't been for that, he would have slept alone every night. When he gathered his Gryffindor courage and told her he'd fancied her since fourth year when he'd asked her to the Yule Ball, she only smiled sadly and kissed him, gentle and tender and protective. Sweet Ginny, strong and fierce Ginny, beautiful Ginny… she'd fancied Harry since the age of ten. And who would choose Neville over Harry?
She did, for a year.
For Neville, it was more than just a comfort thing. He never asked if it was the same for her. He just took what he could get.