It's been a looooonnnnnng time since anyone asked my opinion on dating. Given my personal history, can you blame them? But I do have one friend who occasionally seeks my counsel. (Or, to use their words, "WWPCD: What would La Poutine Cachere do?".) The current conundrum facing my erstwhile friend was whether it is appropriate to have friends of the opposite gender if you're dating someone seriously. The foregone conclusion the said friend arrived at was that (surprise!) it's halachically better to abstain from such relationships. The following is my take on the issue.
In my case, I don't buy into the typical frummie adage that all opposite gender friendships de facto carry a sexual undercurrent, since I have personally had purely platonic friendships with men. Yes, there definitely are people out there who cannot think of someone of the opposite gender except in a sexual way, or who cannot refrain from wanting to "keep the door open". However, such people typically give off signals that they feel/think that way, and one should refrain from friendships with such people. Granted, I chose to only be friends with men who do not fall into the above category.
Another point where I differ from the party line is that I don't feel that maintaining opposite gender friendships is counterproductive to maintaining a relationship with one's significant other. Your significant other must always remain your primary relationship, and the focal point of your life. However, I believe that having friends outside this primary relationship is beneficial to that relationship, since one should rely on themselves to address their own "needs", i.e. they should not expect their significant other to be their "everything". As I tell every man when dating, my philosophy is that you are two independent people in a relationship, and is obviously the relationship to which one devotes the vast majority of one's energy. But if you devote all of your energy solely to your relationship without devoting adequate energy to also taking care of yourself, you will inevitably become dissatisfied in your relationship. Indeed, you may come to even resent all the effort you are devoting unless you maintain interests and a sense of independent identity.
Friends are a part of this separate identity, and if your friend happens to be of the opposite gender, that isn't unequivocably a problem. Where such relationships typically become a problem though is where there is either a lack of trust or a lack of self-esteem. After all, if your significant other trusted you implicitly and had adequate self-esteem, such relationships shouldn't fuel any shalom bayis problems.
That said, human nature being what it is, I suppose the rebbeim instituted the party line because people do have trust and self-esteem issues. Consider the party line therefore a pre-emptive measure to prevent potential marital discord. I can certainly respect that. In the end then, you need to hold where you're comfortable. My only caveats are that one should ensure that one isn't giving up such friendships due to trust issues, and that such abstinence will prove fruitful long-term. The goal, to reword, is promote shalom, through whatever avenue works for both you and your relationship with your beloved.