There are risks in not discussing it and assuming that your partner agrees with you. Some people who think they are in a monogamous relationship, such as a marriage, find out that their partner has had sex with others.

Both monogamous and open relationships can bring benefits and challenges. For example, some couples in monogamous relationships say they enjoy feeling both physically and emotionally committed to only one person. However, they may feel frustrated if they have a higher or lower sex drive than their partner.

Some couples in open relationships say they enjoy the sense of freedom and variety it can bring, but it can also highlight any feelings of jealousy or insecurity within the relationship. Mutual trust and honest communication are vital in both monogamous and open relationships.
If you both agree to be monogamous it is important to discuss what would happen if either of you broke this agreement. If either of you feel that you must hide the fact that you've had sex outside the relationship, that can seriously threaten your relationship.

One advantage of monogamy is that no sexually transmitted infections (STIs) can come into your relationship
If you have HIV, an STI can be harder to treat and, until it is treated, your HIV 'viral load' increases. Infections that can come into an 'open' relationship include syphilis, herpes, chlamydia, gonorrhoea and hepatitis C, a serious liver infection caused by a virus that some people with HIV have got from sex. There is no vaccine for hepatitis C, it can cause serious liver problems, including cancer, and treatment can be difficult and unpleasant. Re-infection with a different, more damaging strain of HIV is also possible although it is not clear how often this happens.

Using condoms with people outside your relationship will make it less likely that you'll pick up infections (and give them to your partner) although some STIs can be passed on despite using condoms and through oral sex.