I think Tsunade was in love with Jiraiya only that she noticed too late.
I mean, also, Jiraiya was always a pervert and a jerk... but I think he was like that due to the lack of the person he really loved. He was probably looking for a Tsunade in all those women he met, or "dated".
I find it really touching when Jiraiya is about to die and he feels sad for the big failure he thinks he was. He mentioned "always rejected by Tsunade" among one of his biggest fails in life. That is just not fair!!
The Pervy deserved an opportunity because he really loved Tsunade even after all those years, being both so old, he still loved her.
Tsunade was always too tough with him, she only showed some love when they talked for the last time. I was now watching that moment again in the anime and I cried like a fool . Tsunade realized she was about to lose the person that really loved her the most in the world. She meant the world to him and I think he wanted to go alone to fight Pain not only to protect Konoha, but to show Tsunade that he was really worthy and not just a perverted old man.
They both would have been a lot happier in life if they happened to be together. Even when he was a pervert and she was an alcoholic gambler... I think they were the perfect match. After all, the old Sage died frozen in time, yearning forbidden wishes.