Is Sucking allow in Islam
In general, Islam allows husband and wife to pleasure each other in every way except those things which are clearly forbidden such as anal sex and sexual intercourse during menstrual cycle.
In regard to oral sex, it would certainly fall under the category of permissible acts but it may be considered Makrooh to some extent according to some scholars. The term oral sex is a very open term that can be defined differently by different people. If the definition of oral sex does not pertain to any acts clearly forbidden by Islam then there is nothing wrong with that but if it pertains to some activity that is not approved by Islamic Shari'ah then one must refrain from such activity.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America