All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The mentioned supplication is one of the recommended manners of intercourse which the Sharee‘ah has guided us to. It is only recommended, not obligatory.
In Al-Mughni, Ibn Qudaamah said:
It is recommended to say Bismillaah (In the Name of Allaah) before intercourse because Allaah The Almighty Says (what mean): {And put forth [righteousness] for yourselves.} [Quran 2: 223] This means saying "Bismillaah" when having intercourse as stated by ‘Ataa’. Ibn ‘Abbaas narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever says (Bismillaah; Allaahumma Jannibna Ash-Shaytaan wa Jannib Ash-Shaytaan ma Razaqtana (in the Name of Allaah; O Allaah, Keep the devil away from us and Keep the devil away from what You Would Grant us (the baby), when having intercourse with his wife, and they gave birth to a baby, then the devil will never afflict it." [Al-Bukhari and Muslim]
In Al-Majmoo', An-Nawawi said:
Saying Bismillaah is recommended in Wudhoo’ (ablution), all acts of worship as well as other deeds even on having intercourse. This is explicitly stated by Al-Qaadhi Abu At-Tayyib, his companion Ibn As-Sabbaagh, Shaykh Nasr and others. Shaykh Nasr said, "It is also recommended when one leaves his home. In His Saheeh, Al-Bukhari named a chapter indicating that saying Bismillaah is recommended in all cases including intercourse. He quoted the Hadeeth of Ibn ‘Abbaas."
Consequently, you should be keen on saying this supplication. However, there is no blame if anyone does not say it.
Allaah Knows best.