بسم الله الرحمن الرحيم
The holy Quran says:
أُحِلَّ لَكُمْ لَيْلَةَ الصِّيَامِ الرَّفَثُ إِلَى نِسَآئِكُمْ الآية [البقرة : 187]
It is made lawful for you, in the nights of fasts, to have sex with your women.
The verse tells that it is lawful to have sex with wife in the nights of fasts. Hence if someone wants to have sex with his wife during nights then he may do so it is wrong to consider it unlawful and bad. Yes, during fast it is unlawful, to have sex with wife it shall nullify the fast and kaffarah shall be wajib. Hence it is necessary to avoid it while fasting.
Allah knows Best!
Darul Uloom Deoband