I think men (or women, for that matter) who are attracted to pre-op trans women are far, far too demonized by our community. Here's why I think so:
The person is attracted to a woman with a penis. That isn't inherently wrong, just as it isn't wrong for someone to be attracted to the same sex. The person does not get to decide their sexuality.
The second stage would be the personality. Is the person objectifying their point of attraction with no interest in/respect for the trans woman's humanity? If so, then this is a problem. If not, then it is the same as anyone else seeking out a partner who fits their sexual needs.
Since it has been established that objectification is a problem, let's move onto the alternative. The man (or again, woman) wants to enter a relationship with a pre-op trans woman and enjoy her company, respect her needs, and so on. I think this is completely fine.
Finally, the question comes up of how the man/woman will react if the trans woman desires GRS. I think it's unfair to demonize someone who would not want to stay in the relationship -- the same way it's not necessarily fair to demonize someone who feels uncomfortable with a transitioning partner. It disregards the man/woman's sexuality, and makes it difficult for them to fulfill the needs they initially set out for. I think, with that in mind, it would be wise to discuss wishes for both parties when entering a relationship. If there is a desire for GRS and the partner opposes it, then it may not be a good idea to proceed; in that same vein, it shouldn't be assumed that all trans women have a problem with retaining their initial genitals. Not all trans women mind having a penis.
It would be an issue if one party lied about their desires or their intentions during the relationship's onset. Otherwise, I think there is no issue, and that humans should be free to pursue happiness -- it is a question of individuality.
Edit: some of my prose was irritating me.