Question we have to ask, what is a stable relationship. Your saying your two 18 year old roommates have boyfriends, but, they really do not have a wish or thoughts of their boyfriends will turn into a husband and wife relationship. My theory of a stable relationship, is being with someone for two years, and it is rational to say it can lead to a marriage.
I am 29 years old, and if I was looking for a boyfriend: I would be more interested in a man who has gone months and maybe years without a girlfriend. Reason being, I would find him to be a more secure man because he has been alone. You can find a boyfriend someday, and I think he will find you more interesting and more secure as a woman because you never been with a collections of past boyfriends and a need to have a boyfriend.
We know the women who have a collections of boyfriends on their cellphone, and it is a Monday, and she is unsure who she is going on a date of Friday. Yes she is popular, but, to be frank she is also very insecure. Yes, your feeling insecure because you do not have a boyfriend now. Still, when you do get a boyfriend, he would think of you as a secure woman. Because he will feel you want him as a boyfriend, not because he feels you need to have a boyfriend. There is a difference.