We have to wash our hair thoroughly whenever we offer a ritual bath (ghusl) to remove ourselves from a state of major ritual impurity (janâbah). We must do so whenever we need to in order to offer our prayers on time. We do not have to do so every time we engage in sexual intercourse.
There is a concession in Islamic Law if a woman has braided hair. She does not have to undo the braids if the ghusl is on account of sexual intercourse. However, she still must wash her hair.
It was related by the Mother of Believers, Umm Salamah, that she asked the Prophet (peace be upon him) about a woman’s bath (ghusl). The Prophet (peace be upon him) told her: "If a woman is performing ghusl after having sexual intercourse, then there is no need for her to unbraid her hair. It is sufficient that she pours water over her head three times. But, when she is performing ghusl after completing her menstrual period, then she has to unbraid her hair." [Sahîh Muslim]
Again, you do not need to perform ghusl every time you have sex. It is only required when you need to offer an obligatory prayer after having had sex. You should try to persuade your husband to choose times when you will not be forced to take many repeated baths during the day. For instance, the time after the night prayer all the way up to the time of the dawn prayer is open. There are no other obligatory prayers to be offered within this long interval of time.
Also the time between the dawn prayer and the noon prayer is quite long and is uninterrupted by any intervening obligatory prayers.
And Allah knows best.