I have noted the following analogies regarding the OP question:
I suppose oncologists have no idea about cancer, either.
You do not need to do drugs to tell others not to do them. You do not need to be sick in order to be a Dr. You do not even need to “play the sport” in order to coach.
Who is in a better position to assess the effects of drug use?
Except for the “coach” analogy, I would advise against using these as they are all “negative” analogies. That is to say, cancer is a natural evil, and drug use may be a moral/natural evil (depending on the drug). You wouldn’t want to give the impression through analogical implication that Catholics think sex is evil.
Moreover, a very reasonable response to all of these analogies, including the “coach” analogy, is that, sure you don’t need personal experience to give advice or have knowledge, or even expertise, but having personal experience gives you a unique perspective that will make your advice more relevant. For example, a coach that played basketball for twenty years can be a much better coach than one who’s never played. A “recovered” addict can provide more relevant advice because they know what the addict is going through, and can advise how they got through it. An oncologist who survived cancer may be more impassioned for their work, as well as more empathetic to their patients than one who has no personal experience with it.
It’s a common human understanding that practical experience has a very different worth than theoretical knowledge.
There are a couple of problem with your girlfriend’s question. One is that it assumes a disconnect between sexual intercourse and genuine human love. Two is that it assumes living celibately somehow means th priest becomes non-sexual. Neither of these are true.
First, sexual intercouse is a particular expression of genuine human love. And since this is true, one need only to understand what genuine human love is, and to have experience with it, in order to have a correct understanding of proper expressions of it, including the proper expression of it between sexual lovers. Genuine human love involves a lover, a beloved, and love (the desire for the objective good of the other). Priests certainly experience this, and thus this experience may be drawn upon to talk about the unique form of love between spouses.
Secondly, priests are sexual. I don’t mean “sexually active,” as you might think. Rather, I mean that they have a sex. They are male. They are men. A priest is a “he,” and everything he does is done as a man does it. Thus, if a priest has a proper understanding of his maleness, what it means to be a man, then he has particularly relevant experience to offer other men, even if the men he’s giving advice to aren’t priests themselves. That a priest is celibate doesn’t mean they don’t have the same basic human desires as other men.
Furthermore, as a member of the Church, the Body of Christ, every priest, like every other Christian, is “Bride” to Christ. This isn’t mere analogy. The Eucharist is the “bedroom” of the Church, so to speak. Just as a husband “goes into his wife” in sexual intercourse, and they become “one flesh,” so too Christ “goes into us, His bride” in Communion to become “one” with us, and through this union the life of the Holy Spirit comes to dwell within us. By learning to be a loving spouse to Christ, a priest gains some insight that can be very relevant to a woman’s relationship to her husband. And this in turn strengthens his understanding of his own manhood, enabling him to be a better priest to his congregation.
What we as married people need to understand is that the unique expression of love that we are able to share with our spouse is an image of the eternal relationship that we will have with God in heaven. It is an act of total self-giving. That is what genuine love is. And though a celibate priest does not experience this particular expression of it, he may certainly have experience with total self-giving in other loving relationships in his life, particularly in his relationship to God.
It is not true that “having sex” makes you more qualified to talk about love. Actually, having love makes you more qualified to talk about sex. That’s the great value of the Theology of the Body. Sex is an expression of love, therefore anyone who has genuine love can have a proper view of sex. Love comes first. Unfortunately, our culture says sex comes before love. But that’s like saying opera comes before music. Well, it’s just backwards. Anyone who has sound grounding in the basic principles of music is better qualified to talk about the various expressions (genres) of it, than some who grew up exposed only to operatic music. Such a person would find other expressions of music strange and distasteful, and their view of, say, country music would be skewed as they tried to apply certain principles of opera to country. They would just not get it, and would probably dislike it.
Well the same is true about love and its various expressions. If I have a good understanding and experience of genuine love, then I would be qualified to talk about its various expressions, such as parental love, brotherly love, spousal love, neighborly love, patriotic love, etc. But if my starting point is sex, and I don’t have experience of genuine love, then my view of all these other forms of love will be skewed and colored by it. I wouldn’t be able to have proper loving intimacy with another man without thinking it was somehow homosexual. I wouldn’t be able to have proper loving intimacy with my mother, or my daughter, without thinking it was somehow incestual, I wouldn’t be able to have a friendship with another woman without thinking she was flirting, etc.
Love first. When you know this, then you can speak about its various expressions.