There is a minhag not to have marital relations on Rosh Hashanah, in keeping with the awesome nature of the holy days and because the time is better spent in prayer and repentance. This usually does not apply when it is a mitzvah to have relations, such as on mikveh night, or to fulfill the commandment of p'ru ur'vu ("be fruitful and multiply"), which entails having at least one boy and one girl.
Abstinence on Rosh Hashanah is merely proper conduct; it is not a law. Newlyweds may not wish to or be able to refrain. There is also local custom to be considered.