There are a lot of reasons that couples stop having as much sex as they used to. It doesn't always mean that your boyfriend is gay! However, if he starts talking about wanting to explore his sexuality and mentions that he prefers men more than women, this could be a sign that he is bisexual.
Does he have a lot of friends who identify as gay? Although a lot of straight guys may have some gay friends too, if your boyfriend's social circle is predominately made up of LGBTQ people then this could be a sign that he's gay.
Do you notice that he enjoys doing things that are culturally girly like going shopping for clothes or making sure his hair is in place? While it is very normal for boys to be interested in these types of things, if your boyfriend goes out of his way to do them and constantly criticizes the clothing you wear then this could be a red flag.
Is your boyfriend always bringing up how judgmental his family, town and society is about gay people? This can be a really big deal for someone who is gay, especially if they're not ready to come out yet.
It's important to remember that the only way to know whether your boyfriend is gay is to ask him. It's also worth remembering that a person's sexual orientation can't be determined by their appearance, clothes, or the way they talk.