it had been heavily hinted that the two were together for quite some time, but they both denied it as nothing. and then in shippuden after asuma's death it was shown that kurenai was pregnant with his child, thus confirming their relationship.
also i find this whole scenario suspiciously similar to shikamaru and temari's. the two are always together, deny dating each other, but yet continually are seen as together and are happy for the most part.