There is nothing explicitly mentioned in the books regarding Robb's love life prior to Jeyne Westerling but there are implications that go both way.
First of all we have to see his relationship with Theon Greyjoy. Theon and Robb were best friends and as close as brothers to each other. Theon was no stranger to bedding serving girls and he and Robb apparently used to discuss the girls afterwards. The passage cited below, describing one such instance.
Two serving wenches stood beneath the sign of the Smoking Log, the
local alehouse. When Theon Greyjoy called out to them, the younger
girl turned red and covered her face. Theon spurred his mount to move
up beside Robb. "Sweet Kyra," he said with a laugh. "She squirms like
a weasel in bed, but say a word to her on the street, and she blushes
pink as a maid. Did I ever tell you about the night that she and
"Not where my brother can hear, Theon," Robb warned him with a glance
Bran looked away and pretended not to have heard, but he could feel
Greyjoy's eyes on him. No doubt he was smiling. He smiled a lot, as if
the world were a secret joke that only he was clever enough to
understand. Robb seemed to admire Theon and enjoy his company, but
Bran had never warmed to his father's ward.
AGOT - Bran V
It looked like Theon had forgotten that Bran was with them and Robb didn't want him to listen to all the adult talk. Now this can be interpreted both ways. One could be that they used to share their exploits with each other. Or that Robb was that one virgin friend who loves to listen his best friend talk about his sexual conquests. Jon Snow was Robb's best friend too but Jon's much more solemn in nature and has an aversion to anything that leads to making bastards due to his personal experiences, so it is understandable why he was not included in these talks. Not to mention, Jon never got along with Theon.
Then we have Catelyn's account which implies that serving girls in Winterfell were very much attracted to Robb, the young heir to the North.
Robb got to his feet slowly and sheathed his sword, and Catelyn found
herself wondering whether her son had ever kissed a girl in the
godswood. Surely he must have. She had seen Jeyne Poole giving him
moist-eyed glances, and some of the serving girls, even ones as old as
eighteen … he had ridden in battle and killed men with a sword, surely
he had been kissed. There were tears in her eyes. She wiped them away
AGOT - Catelyn XI
They were not the only ones, Princess Myrcella kept staring at Robb during the feast at Winterfell and Robb was grinning like an idiot when he got to escort Myrcella to her seat prior to that. It was as if he (13?) had never had a girl (8-9?) touch his arm before. But just the fact that Jeyne Poole or serving girls were interested in him doesn't really say if Robb returned their affections. Nor does the thing with Myrcella who was way too young and probably didn't understand all that anyways.
Finally we have his one explicit sexual encounter, that was with Jeyne Westerling.
"I took her castle and she took my heart." Robb smiled. "The Crag was
weakly garrisoned, so we took it by storm one night. Black Walder and
the Smalljon led scaling parties over the walls, while I broke the
main gate with a ram. I took an arrow in the arm just before Ser Rolph
yielded us the castle. It seemed nothing at first, but it festered.
Jeyne had me taken to her own bed, and she nursed me until the fever
passed. And she was with me when the Greatjon brought me the news of .
. . of Winterfell. Bran and Rickon." He seemed to have trouble saying
his brothers' names. "That night, she . . . she comforted me, Mother."
Catelyn did not need to be told what sort of comfort Jeyne Westerling
had offered her son. "And you wed her the next day." He looked her in
the eyes, proud and miserable all at once.
"It was the only honorable thing to do. She's gentle and sweet,
Mother, she will make me a good wife."
ASOS - Catelyn II
This folly seems to imply that Robb was not used to bedding women and then leaving them at all and Jeyne might as well have been his first. He risked his life, his family and his kingdom for the "Honourable thing", something you don't expect a man, well let's say more experienced in the ways of the world, would. Of course there's a possibility that he genuinely fell helplessly in love with her, but he doesn't mention that emphatically when he gives his reasons to his mother. If it was all about love, he would not have mentioned how she comforted him and he had no other choice. It is also possible that Jeyne was the first noble woman he had laid with, the first one with whom he could hope to have a future so she was different from serving girls and peasants. But it still doesn't make sense to lose everything for her. To quote Jaime:
She did not look dangerous. Jeyne was a willowy girl, no more than
fifteen or sixteen, more awkward than graceful. She had narrow hips,
breasts the size of apples, a mop of chestnut curls, and the soft
brown eyes of a doe. Pretty enough for a child, Jaime decided, but not
a girl to lose a kingdom for.
AFFC - Jaime VII
That's what Jaime says on the matter, a guy who is hopelessly in love and has never been with anyone except one woman and even he realises how much of a folly was it. That's absolutely not something you expect from a man who is used to do that.
So in conclusion, there is no conclusive and explicit evidence for or against this. Robb may or may not have been a virgin. The evidence, or lack thereof, compels me to argue that until better evidence appears, we should assume that he was a virgin.