You should avoid any sternous activity including sex till you are fuly healed. Only your surgeon can tell you when you have achieved that threshold. Then there are many variables in technique of sex that effect your question. It is best that you have an open disussion with your surgeon and seek personal guidance and advice regarding sex.
Allow time to heal before resuming exercise and sexual activity. In general, my patients resume sex when the tapes and sutures have been removed. Check with your surgeon.