Is it okay or sinful to date a seminarian, even though there’s no sex?
Yes, it is sinful since this could actually become a stumbling block in his response to the call to priesthood. I have seen a number of people have issues because they are attached to a girl, sometimes they don’t know how to break free and most times this negatively interferes with their discernment process. So yes, it is sinful to be in this sort of relationship.
Another thing to consider is, why are you dating at all? If you are not dating someone for marriage, what is the point of the relationship to begin with? It can only dispose one to temptations or inordinate thoughts. Seeking exclusive relationship with someone should be only for marriage.
No vocation is an end-in-itself, but all a means to reach God and do his will on earth. Each vocation, no matter how wonderful the outward appearance becomes horrendous when they prevent us from discerning and doing the will of God. Anything at all that derails one from finding their vocation on earth and consequently finding the peace and fulfillment that comes with it is evil.
So, you have to understand that the search for our “happily ever after”, isn’t a matter of passive feeling or infatuation. Nor is it on some meaningless “crush”, it is a very grave thing. So, whilst i can say that preventing anyone from finding their calling is sinful (no matter the vocation), i can also say that everyone needs a spiritual director especially when trying to enter this initial discernment. Maybe what you have isn’t love, if it prevents another party from being fulfilled. Maybe it is love, but disordered or misdirected. Whatever it be, it can become dangerous when not properly guided. So, talk to someone if you feel something for a seminarian or a priest or any clergy or married person. This is not good at all and needs to be dealt with before it hurts someone or damages a life.