How far is too far? If you are asking this question, my friend, you are starting with the wrong one. Before I tell you what question you should be asking, there are a few basic items I want to cover first.
Remember that God will judge us by our conscience and intentions. So the first steps need to involve understanding what our faith teaches and why. We must make sure that we have a well-informed conscience. This blog post won't provide a foundation for understanding Church teaching.
If you need to learn about what the Church teaches (and why), I would recommend you ask some Catholic friends for a good priest to talk to. If that is not possible, I have no doubt that the women of His Own will be able to assist and provide referrals.
Let's assume you have informed your conscience on what the Church teaches and why. This means you also understand that the pleasure of sex is not the primary reason for getting married. It is a good thing and it is normal and healthy that you have an urge to do that. God made you this way, so don't feel guilty about it.
However, these urges and desires need to be channeled toward a holy purpose and action. You might find yourself wondering how this is possible. To understand how this is possible, we need to know what authentic love is. For that, we can look to Scripture for insights. The best example of love in action is Our Lord's Passion. What we learn from His example is that authetic love demands self-giving and sacrifice.
St. Paul breaks things down even more for us in Ephesians 5:25 which states: "Husbands, love your wives, just as Christ loved the Church and gave Himself up for her." Jesus was mocked, beaten, dragged His cross, crashed on the ground multiple times, kept getting back up, and was crucified.
He had the chance to avoid all this by denying who He was in front of the Jews and in front of Pilot. He didn't try to to find the easy way out. He didn't ask,"What's the farthest I can stretch this situation to cause a more favorable or pleasurable outcome for Myself?" Men, here is your example.
Let's review what we've just learned. Sex (and the desire for both sex and pleasure) is a good in itself as God made us this way. The trick is to not fall into lust, but to love women authentically as God intended. We learned that authentic love is self-sacrificing and requires a man to "die to himself" as Christ sacrificed Himself and died for the Church.
Most of us men won't be asked to suffer a physically brutal death like Jesus. We are called to sacrifice and and put our spouse or significant other's needs and desires ahead of our own. That is what it means to "die to self". The purpose of life, gentlemen, is to get to Heaven. Marriage is a means of becoming holy (believe it or not).
The first question we should ask is: How do we help the girl (as well as ourselves) grow in holiness? How do you help each other choose authentic love? How far can I go to love in a self-sacrificial way as Christ did? How can I serve her?
You might be wondering what this looks like. Some examples or best practices include: when there is a sexual urge in your head, remind yourself that this is an opportunity to choose love. Instead of acting on lust, do something for her that is sacrificial. A powerful prayer I've found is "Jesus, help me to be Your love and Your touch to her."
Maybe her car is incredibly messy all the time and it drives you nuts. Go clean it for her (and don't expect or ask for gratitiude). Maybe her roommates left dirty dishes all over. Wash them or put them in the dish washer. Go to adoration with her.
Maybe she struggles with purity as much as you do. Then try to do activities in public. You can also have plans to do things later in the day so that you don't spend too much time alone and create temptation. Incorporate prayer together as a regular part of your dating life.
A few last thoughts: Men, if you are doing this right, you will find that the girl is even more attracted to you. If you are consistent, she will see that over time and love you for it.
But also, as men, it's important to check our intentions, making sure we are doing something out of selfless love, not to get something from her. Keeping a purity of intention is important.
Think of it this way: women love flowers. I think it might be because they share some core common traits. Flowers need appropriate amounts of water and sunlight to grow and bloom into their potential. While women like sunlight and water too, we as men can provide them with authentic love and affirmation.
In my experience, when a woman receives authentic love and affirmation, it is a source of stability and strength for her to bloom into a stronger and healthier version of herself. An awesome side effect of this is that she will have a similar impact on you as well.