Dear Fr. Nielsen,
My first piece of advice is this: in all likelihood, a woman who comes to you seeking advice about dating and relationships already knows the answer to her question. At the same time, she might be overwhelmed by fear, anxiety, and doubt, which can complicate her decision-making. What she probably needs from you most is to be reassured that her feelings are valid, and that everything will be okay - however this particular relationship turns out. In addition to counseling her yourself, it would be helpful to connect her to single and married women of faith who can relate to her and provide guidance.
I approach my response assuming that we are addressing women of faith who believe that the teachings of the Church are true and good. The woman in question is already convinced that masturbation, pornography, and casual sex are harmful to the human person and to relationships. However, she might struggle with rejecting a potential life-partner because he engages in one of those things.
Let’s pretend that a woman meets a man and, in all other regards, he seems like a great match; but this issue is his “red flag.” As with all other red flags, the woman is concerned that this issue will cause significant strife and could be disastrous if she pursues the relationship. The vital question concerning red flags is the man’s attitude towards the issue: is he nonchalant about it or does he defend it as something good?
In guiding the women who seek your advice, encourage them to keep four things in mind:
1. You cannot expect a man you are dating to change for you.
Unless a man wants to change on his own, he likely won’t, and the situation will end in heartbreak. A man should not be a woman's project and, if you are going to marry someone, you should love them as they are instead of wishing a major part of them were different.
What if you aren’t concerned with changing him and plan to just “deal” with the sin or bad habit he is unwilling to change? You might be like Servant of God Élisabeth Leseur and love him through his sinful ways until he miraculously has a conversion of heart. Perhaps this will work out for you, but keep in mind:
2. Sin is destructive; it causes harm and sexual sins, especially, attack marriages.
A recent study that surveyed 2,000 couples about their relationship satisfaction and use of sexually explicit media found that American couples who began using pornography were roughly twice as likely to get a divorce . Furthermore, these issues will impact both you and your future children. They deserve to have the best father that you can find for them - and you deserve to have the best husband that you can find.
However, if the man agrees that masturbation, pornography, and casual sex are harmful and sinful, but he still struggles with them, this is a different situation.
3. Selection or rejection of a potential spouse must be based on his character, core moral belief system, and faith.
We are all sinners, grappling with sinful habits or actions that hurt us and those around us. However, “we do not have a high priest who is unable to sympathize with our weaknesses, but one who has similarly been tested in every way, yet without sin. So let us confidently approach the throne of grace to receive mercy and to find grace for timely help” (Hebrews 4:15-16). If we reject sin and turn to the sacraments and prayer to overcome it, then we shall be victorious, “for whoever is begotten by God conquers the world. And the victory that conquers the world is our faith" (1 John 5:4).
Finally, we must remember that, although we might desire to be married and have a family, if we settle for the wrong man, it won’t end in the life-giving joy we seek.
4. Above all else, trust in the Lord.
Even when the Lord places a desire for marriage and a family in our hearts, He does not call us to be bull-headed and take what we believe to be ours. He calls us to come to Him, to let Him satisfy all of our needs and desires, and to receive His love and gifts according to His plans, not ours. And no matter how challenging or incomprehensible they seem, His plans and His ways will bring us so much more joy and fulfillment than anything we could achieve on our own.
 Perry, S. L., & Schleifer, C. (2018). Till Porn Do Us Part? A Longitudinal Examination of Pornography Use and Divorce. The Journal of Sex Research, 55(3), 284-296. doi:10.1080/00224499.2017.1317709
This author would like to remain anonymous.