Choose a calm, private moment away from the bedroom, open with appreciation, speak from your own experience with "I" statements, ask more than you tell, and keep the first conversation short. Treat it as the first of many talks rather than one big negotiation.
Most couples experience different levels of desire at some point. It is common, rarely anyone's fault, and often shaped by stress, health, and how desire works for each person. Understanding spontaneous and responsive desire, reducing pressure, and checking for medical factors usually help more than trying to match exactly.
Consent means freely and enthusiastically agreeing to intimacy, every time, with the freedom to change your mind at any moment. Being in a relationship or married doesn't change that. In practice it means checking in, noticing hesitation, making "no" easy to say, and stopping whenever someone wants to.