If a partner is not loyal, a relationship quickly becomes unworkable. People do recover from an affair but usually need professional help to do it. Make a commitment to be faithful and stick to it. If you’re not happy in the relationship, get counseling and not a part-time lover.  If you’re faithful to someone, it means you’re faithful on all levels. This means physically but also emotionally. Some people think it’s okay to establish intimate bonds even though they may just be spending time with another person and nothing else, but it’s really not. It will eventually cause problems in your relationship. Be clear with appropriate boundaries as much as possible. What is appropriate varies from culture to culture, and often with one's age. But it all comes down to understanding what is respectful, clear, and comfortable to ask from a romantic relationship. Dating a person once is not a committed relationship, for instance. If asking a person out on a date, make sure that is clear; it can be awkward if a woman is not entirely sure she is at a restaurant as a friend or as a date. Be clear on if you see your sexual relationship as more casual or more serious. Some people are comfortable with "booty calls" and "friends with benefits" types of relationships. Others see sexual relations as a very deeply emotional act done only with someone committed to a relationship (or after marriage). There are wide varieties of behaviors considered a "committed relationship," from traditional marriage, to "live-in boyfriend or girlfriend", to more radical "open marriages," and polyamory. A person seeking a traditional marriage may be disappointed if the partner is looking for something else. Likewise, someone seeking something else may be disappointed if the partner is looking for a traditional marriage. Trust builds in an atmosphere of security and safety. The cycle of hurting each other, either verbally or physically, and then rejecting the other person, creates a lot of fear; which undermines trust. Furthermore, trying to...

TL;DR in Vietnamese:
Luôn chung thủy. Cho người yêu không gian riêng và nuôi dưỡng sự tử tế. Yêu người ấy không vì động cơ thầm kín nào khác. Đảm bảo mối quan hệ của bạn là ưu tiên hàng đầu. Luôn ở bên nhau.