It is okay to have sex with half a working heart as long as both people having sex are the legal age. In England, Scotland, N. Ireland and Wales that age is 16 in the republic of Ireland it is 17. If you and your partner are of legal age in your area and want to have sex you are safe to do so with half a working heart.
As with anyone looking to start a sexual relationship a conversation is a good idea, especially with half a heart. You have less energy, so you need to take this into account and warn your partner beforehand.
If you are of legal age and thinking about sex is important that you look into the correct type of contraception for you as there are some you cannot take.
For more information about this check the link below:
LHM Sex and relationships with half a heart booklet