I definitely disagree with that initial assessment. There is enough wrong in this fallen world that, though all are likely called in a natural sense (look at how programmed our bodies are for marriage!), it is way too far to say that it is an expectation for everyone, even among the non-religious. Too many ways to accidentally miss the boat (or even intentionally, but with later regret).
The two general “rules” in the situation you describe above are to avoid near occasions of sin and the golden rule, i.e., treat others as you would have them treat you were the roles reversed. Is being alone with an attractive female an occasion for grave sin for you? It might be for many, but if not for you, then you pass general rule #1. Even if it is a near occasion of sin, since dating is a usually-necessary first step to marriage, there is some slack allowed here on the near-occasion angle (you can risk a little), as long as you are prepared to make amends and a good confession if you fall). From then on, to the extent you inculcate friendships with the opposite sex, try to make sure they understand your intentions to the extent you would expect a normally-intelligent person in her shoes to so guess them. For example, it would otherwise be unfair to a woman if a situation were to develop where she is showing signs of romantic love, while you have no intent of exhibiting same, and you continue to let the person go on acting that way.
But, if you don’t mind my asking a personal question to help us cut to the chase, what exactly is your point with these hypotheticals? What factors relating to your own life are important in applying their lessons?