Having sex
Having HIV doesn't mean you can't have a satisfying sexual relationship or will automatically infect your partner(s). It just means you need to adopt sexual practices that will protect your partner(s) AND yourself.
Evidence has shown that people with HIV who achieve sustained viral suppression to an undetectable level by antiretroviral drugs have no chance of passing on the virus through sex.
However, if you get another sexually transmitted infection (STIs), it may affect your HIV condition and its treatment may also affect your HIV drug effect. You do not want to be found out having STI again when follow up in clinic.
Although viral load maintained at an undetectable level for at least six months have a negligible risk of transmitting HIV sexually, the duration between starting treatment and become untransmittable need a period of time. When your viral load is still detectable, in addition to using a condom, your partner may consider to use pre-exposure prophylaxis (PrEP) to prevent HIV infection.
When two HIV-positive guys with different virus subtype, having unsafe sex when viral load is still detectable, there will be a chance their HIV viruses will infect each other, this is called HIV superinfection. The new strain of HIV can replace the original strain or they may mix and produce a new mutant. For some people, superinfection may cause them to get sick faster because they become infected with a new strain of the virus that is resistant to the medicines they are currently taking to treat their original HIV infection.
Therefore unless both of you can maintain viral load undetectable for a long time, you should use condom.
If you don't know your partner's HIV status, you probably don't know if he has other sexually transmitted infections (STIs) as well. So always use condoms to protect both yourself and your partner. Better still, talk to him, ask him to get tested, discuss about safer sex practice and get mutual agreement especially if you two plan to have long term relationship.