Policies at Catholic colleges regarding co-ed dorms and relationships vary depending on the Specific college. Not all Catholic colleges have coed dorms, but those that do tend to have policies that reflect the Catholic teachings on relationships, purity, and chastity.
Typically, even when a Catholic college offers a co-ed dorm living situation, is still means separate living quarters by gender. This could be a co-ed building, but gender-specific floors or suites. Sometimes they might have designated co-ed areas, where there is a living space that is shared by both genders. This could be like a living room and kitchen area that are shared. But even then, the specific rooms or living quarters are still private or gender specific.
There are almost always rules about visitation and "open hours" when someone of the opposite gender is allowed to visit or be in the dorm room/living quarters. Each college will have their own specific rules regarding visitation, as well as rules regarding conduct between opposite genders and in relationships.
While the rules will vary between colleges, there are some general guidelines that apply in most cases. Many Catholic colleges will have codes of conduct that prohibit premarital sex, require doors to be kept open when the opposite gender is visiting, and will have rules about PDA on campus. The code of conduct is meant to encourage students to live in a manner that is in line with the Catholic faith and its teachings on relationships and purity.
To find out what the specific rules regarding visitation, co-ed dorms, and relationships are at a Catholic college, search the website for their student handbook or code of conduct, where the values and expectations are outlined for all students.