The terms alpha and beta (and others) are used to describe various types of men and their levels of attractiveness to women.
Sexual attractiveness that is.
There are certain things that are sexually attractive to men, other things that are sexually attractive to women. If all you're looking for is sex, all you need to do is to be the most attractive that you can be.
This forum is very marriage minded (and by marriage we mean a long term committed relationship, not necessarily the legal contract). Marriage is - by definition - a sexual union. However, marriage has certain anti-sex elements to it.
Let's take the hottest woman around, a 10/10. This woman has a perfect figure, perfect skin, perfect hair, perfect breasts, perfect ass, smells nice and has a pretty face. After all, 10/10 means perfection!
Likewise, the perfect alpha male 10/10 in male attractiveness (although more difficult to clearly define) has a lean and built body, he's strong, he's tall, he can get things done, he has ambition, he's successful, he doesn't put up with shit from anyone.
The 10/10 woman will need to watch what she eats and often is very strict about her diet, she may need expensive skincare, hair products and fragrances and may dress in expensive clothing. While all of these keep her sexual value (SMV) high, they lower her RMV, her relationship value because none of these things are conducive to living with another person long term (for most people) and they certainly aren't good for raising a family.
Likewise, a man who needs to spend many hours at the gym to keep himself muscular, expends emotional energy being tough and to pass shit tests - is a net cost to marriage and family. The stereotypical dad-bod is a thing because most fathers prioritize their wives and children over their personal fitness. When life gets hectic trying to support and raise a family, that's the first thing to go.
In plain terms - if you marry a highly attractive man (or woman), especially if you're going to raise a family, sexual attractiveness will likely drop a bit. Being that marriage isn't only about sex, this shouldn't be an issue if your expectations are realistic.
If you expect your man to be alpha all the time, if beta character traits are repulsive to you, perhaps marriage isn't your cup of tea. Perhaps you should ride the cc until you can't anymore. Expecting a man to always be alpha is the same as expecting a woman to always be a 10/10 hot 19 year old piece of ass. The desire is understandable but it's also an unrealistic expectation in the long run.
Good news is, between the far extremes of 10/10 hot gym junkie who costs a fortune in products and emotional energy on one end and the fat and lazy, sexually repulsive hag on the other end - is a happy middle ground.
This middle ground is wisdom that our grandparents understood quite well, but this knowledge has been somewhat lost today. To balance realistic expectations from the other while always striving to be the best version of yourself that you can be. I'm not saying everyone's grandparents were like that, mine certainly were.