I’d say that those of us who maintain friendships with members of the opposite sex have to be realistic.
If you have a friend, your sex or not, who consumes a huge amount of your time, of course that is going to give you less time for an actual courtship. That’s as true of a obsessions with hobby shopping or ESPN as it is a very close friendship, though. When it comes to impinging on the attentions a spouse ought to be getting, hobbies or even volunteer work probably get a pass when an opposite-sex friend never would!
If you have a serious physical attraction to someone who doesn’t or cannot return the interest, you have a problem. The solution is to either decide that you’re going to work your way out of the friend zone or else to dial it back to something more distant, more like a pleasant acquaintance-ship. Spending lots of time and investing emotional capital in a relationship that cannot go where you wish it could is a recipe for disaster. Do not do it. If a marriage to someone else is the impinging factor, REALLY dial it back. Dial it back as much as you think you need to, then dial it back about five notches past that. Take the Sixth Commandment very seriously, including Our Lord’s admonition to read the Sixth Commandment strictly rather than loosely. (Matt. 5:27-30)
Yes, you can have friends of the opposite sex who are just friends. Sometimes, the guy who seems like a brother to you really does feel you seem like a sister to him: that is, the two of you value the friendship but consider a sexual interest something between preposterous and icky. Personal chemistry works against impurity sometimes, too, not just in favor of it. In this case, being realistic involves realizing that transient sexual urges can rush in when you least expect them. These have the power of flood water: even though it looks as if it is only shin-deep, it is very powerful. Expect these feelings when you least expect them, and do not underestimate their power. (If you have been drinking or going through a very emotional time–even a death!!–multiply that by ten or even a hundred.)
Keep yourself out of near occasions of really foolish sin by not spending lots of time alone with someone of the opposite sex not your spouse or family, maintaining what amounts to chaperoning at all times–being in public or where someone might walk in at any time or with third parties present–ESPECIALLY if there will be alcohol involved. Do this with ALL persons of the opposite sex. As it turns out, people are rarely offended at the idea that they could conceivably raise a sexual thought in someone else’s head. “No, I don’t see you that way, but you are not my grandfather. I don’t feel chemistry with you, but someone like me could easily be attracted to someone like you. You’re not chopped liver, you know! People have affairs with someone who was like a brother to them until the day they fell, and it happens more often than you think.”