I'm hiv negative , but want to marry to a hiv positive , and his viral load is undetectable , would i be safe?

HIV: Be aware that undetectable viral load does not mean that the person does not have HIV or that he is cured from the infection. While there is less chance to transmit the virus when the person's viral load is undetectable, at this time no one can guarantee that he/she will remain undetectable for ever. My advise will be to always use barrier protection during intercourse with an HIV+ person

Prevention: There is always the risk of transmission, but you can minimize it. The following website includes helpful tips for both the positive and the negative partner: https://www.aids.gov/hiv-aids-basics/staying-healthy-with-hiv-aids/friends-and-family/mixed-status-couples/. Take care