Answer: Is it against Objectivist principles to date non-Objectivists? Absolutely not.
I think you are referring to the following "...in fact, a man's [i.e., a person's] sexual choice is the result and sum of his fundamental convictions. Tell me what a man finds sexually attractive and I will tell you his entire philosophy of life" (p. 99 in For the New Intellectual paperback).
In the early Objectivist movement, many people understood this principle to mean that one should only be attracted to people who were Objectivists in their explicit philosophical convictions. I think the real meaning of this is deeper and more subtle.
The real philosophy that stands behind a person's character is his sense of life, an emotional, intuitive complex of deep-seated value-judgments and orientations toward the world and others. Most people do not act consistently on their conscious religion or philosophy, and indeed most do not really take abstract ideas very seriously.
What we respond to in others in friendship and romance is founded on sense of life.
What we respond to in others in friendship and romance is founded on sense of life. We don't love someone because they say they are productive, we love them because we see their wonderful engagement with the world and their proactive attitude toward their values. We don't love somebody because they say they are an atheist, we love them for their down-to-earth attitude to life. We don't love them because they say happiness is the ultimate value, we love them because they are joyful and give us joy.
So all the things that would make someone a good candidate for friendship or dating, rest on sense-of-life, not on explicit philosophy.
However, there is an attitude among secular Americans that one can be indifferent to one's lover's religion or philosophy, or that abstract ideas don't really matter. Well, they do matter, because they are what we use to consciously guide our actions. If we take our ideas at all seriously (and total indifference to ideas is not healthy or attractive), we will tend to try to live up to them. Our sense of life will gravitate toward the ideals we prize consciously.
So any serious relationship with someone who embraces ideas at significant variance with Objectivism
would probably be at best uncomfortable for an Objectivist. In my personal experience, I find most Objectivists are happiest in relationships with someone who is in tune with their values. And if one plans to have a family, this can be very important for the upbringing of the children. It's bad enough to have to fight the culture in teaching values to one's children. If one had to fight one's spouse, too, it would be pretty terrible.
This doesn't mean that Objectivists need partners who are card-carrying Objectivists. But they probably need partners who are sense-of-life Objectivists, or what David Kelley calls "Enlightenment Moderns," people with an essentially secular attitude to life who value reason, individualism, achievement, and freedom, even though they may profess a mild religious whim or have doubts about aspects of the Objectivist minimal state ideal.
But these considerations apply to the long term, not to dating. One dates people to develop relationships. If one dates people with good senses of life, then that is appropriate to living by Objectivist principles. If things get very serious, then one will have to see whether the lover's ideas are or can become relatively compatible with Objectivism
. What would be contrary to Objectivist principles would be to commit to someone who was openly at war with Objectivism
. But dating...consider Howard Roark's relationship with Dominique Francon. It's highly stylized, to be sure, but it does represent an Objectivist dating someone who doesn't always agree with his principles, but who has the right sense of life.