This is a very common question!
When a relationship is fresh, people tend to have higher levels of excitement, and so it’s easier to cue up pleasure when you’re more receptive to it. As those excitement levels wane and the monotony of life or routine re-emerges, it is normal and common for libido to dampen.
Part of the reason that so many heterosexual couples find this to eventually be the case is probably partly due to the fact that many cisgender women tend to have “responsive desire”, and many cisgender men tend to have “spontaneous desire”. This means that he’s more likely to randomly feel aroused, and you’re going to need a bit more warm-up to get aroused.
Researcher Emily Nagoski Ph.D can tell you more about this, and I recommend her book “Come As You Are” to any cisgender woman who has ever felt confused about their arousal, or their lack of arousal. Read more about this here.
How do you two focus on intimacy that isn’t sexual or penis-in-vagina? Are you on your cell phones while you spend time together? How often do you surprise each other with a love note, or by help with an errand or around the house? Does your partner ever give you pleasurable touch that isn’t sexual, like playing with your hair or a hand rub? Being able to fuck is something that partnered people tend to take for granted, especially if we aren’t building or reinforcing our connections in other ways. Physical intimacy will often grow when emotional and relational intimacy is nurtured. Ask your partner for what you need, and be mindful of how they respond.
Do you take medication? What kind? Libido can be impacted by SSRIs, allergy medications, and even hormonal birth control. Your doctor might not even know this, because MDs are not often sexual health experts. I recommend that you ask them and do your own research.
And how is your health? Diets high in processed “foods”, fatty meats and greasy eats will clog your arteries and make it more difficult to get blood flowing to all of your organs and cells that facilitate arousal. Get as much sleep, physical activity, and fruits and veggies that you can.
And remember, in order to desire sex, the sex should be worth desiring! Don’t let anybody inside of you unless that’s exactly where you want them to be.