A lot of people don't like to admit it, but sex is very important in a relationship - it can make or break it [the relationship] It doesn't matter how much you love someone, if the sex with them is bad - and is consistently bad - the relationship will sour very quickly. Another thing that some people don't understand is that for people in love, sex is not just "lol naked fun time THIS FEELS GOOD!" Sex can bring two people closer together, it can bond them in a unique way that you have to experience to understand.
It is of no surprise that women are those who refuse having sex in majority of cases since men are known for having sexual urges quite higher. That's why it is known that women fake headache to avoid sex. That being said, why women tend to avoid sex?
I'm going to address this in 2 parts.
1. I definitely think it's still true that men overwhelmingly are the horny ones who have a higher than normal sex drive, but in modern times there are a lot of women who are the sexual ones - more so than guys. However, women tend to be a lot more reserved and subtle about it. Us guys openly talk about boobies, how hot girls are, etc. Women, meanwhile, aren't as open. I think that's what ultimately leads to the misunderstanding that women aren't as sexual as men.
2. For a lot of dumb guys, they think with their dick. They see a hot girl and their dick instantly rises to the occasion and now that guy has a mission: get in her pants. Women learn from a young age that men want sex, it's one of the main things we desire (especially when we are younger). As such, girls learn very quickly that making excuses for no sex is the way to go. This applies for marriage. Guys see their wife in some outfit, get aroused and spontaneously want sex.
I can't speak for all women, though, so I could be wrong. Maybe some guy's sex drive overwhelms their partners, and sometimes, we [guys] may unintentionally make our significant other feel like sex is the only thing that is important. My ex left me for that very reason. That said, I do wish that women communicated, instead of "I'm tired", "I have a headache", "not now". Just be honest and say if you don't want to. If you don't want to have sex or have sex that often,
talk to your partner.
Communicate!