What is monosexuality?
Monosexual pride flag
The first and the most popular monosexual flag was made by the Tumblr user beyond-mogai-pride-flags in 2018:

It has eight horizontal stripes of different widths. Their colors, from top to bottom, are the following: three shades of purple, white, yellow, two shades of gray, and black. The exact meaning of the colors in the monosexual flag is unknown.
History and interesting facts
For a long time, monosexuality (mostly heterosexuality and in some areas and historical periods, lesbianism) was considered the only acceptable norm by society. According to the results of the 1953 research by the revolutionary sexologist Alfred Kinsey, 63% of the surveyed men and 87% of the female respondents said they were "exclusively heterosexual".
Here you can find your place on the Kinsey scale.
More recent studies reveal that purely monosexual identities are becoming less and less common, especially amongst young people. For example, according to the J. Walter Thompson Innovation Group research conducted in 2016 in the US, only 48 percent of the surveyed adolescents see themselves as completely heterosexual.
However, there are still people who believe that monosexuality (no matter whether it is exclusive homosexuality or heterosexuality) is more legitimate or somehow superior to multisexuality or bisexuality. This limiting belief is called monosexism, and it is, essentially, a form of biphobia. In fact, monosexist people view bisexuality as something deviant or morally unacceptable. Obviously, not all monosexual individuals are monosexists. But unfortunately, some monosexual, especially homosexual, individuals still ridicule and dismiss bisexual people because they think bisexuals lack self-acceptance being nothing but lesbians or gays in denial. Other monosexual monosexists see bisexual (but also multisexual) people as individuals who are trying to maintain all the "privileges" of being straight and get into the LGBTQIA+ community at the same time.
Another common monosexist belief about bisexuality, particularly amongst heterosexual people, is that all bisexual individuals, at some point in their lives, leave their partners for a person of the same gender. Some gays and lesbians have similar fears as well - according to them, bisexuals are much more likely to cheat than monosexuals (in this case, homosexuals). It’s definitely time for our society to move past negative and harmful stereotypes like these and become less judgmental and more accepting of all the sexual orientations.
Helping your family and friends understand monosexuality
For people who have been taught that heterosexuality is the only norm, it may be hard to understand that there is nothing wrong with homosexuality or other sexual orientations. For the same reason, it may also be difficult for them to accept the fact that homosexuals and heterosexuals can be kind of in the same group and described with just one term, which is monosexual. If some people in your life react negatively to this word and think that it is nonsense, of course, it is not your task to convince them otherwise. What you can and should do is give your friends, family, and/or coworkers a list of useful resources that will help them understand monosexuality. And who knows, maybe eventually they will actually accept it. Our world is constantly evolving, and so are sexual orientations and their acceptance by society.