Firstly, it is necessary to understand the use of condom. Condom is used to prevent unwanted pregnancy and as a safety measure to prevent Sexually Transmitted Diseases/ Infections and HIV.  Though one have sex with single partner without condom, there is possibility of conception and Sexually Transmitted Infections too. Most important thing is that use of condom is not a barrier in having sexual pleasure hence, it is always safe to use condom even if you have sex with single partner.
For more information about conception and contraception please read articles on links below:  
https://letstalksexuality.com/conception/
https://letstalksexuality.com/contraception/