From Love-shy.com wiki: A wiki for the dating-challenged
Continuum theory posits that love-shyness and involuntary celibacy are not distinct, discrete phenomena, but exist on a continuum, and that it is possible to be both love-shy and incel at the same time. It opposes the majority incel theory and discrete love-shyness theory.
A man could be love-shy, and find it difficult to initiate or respond to advances. At the same time, he could find himself occasionally mustering the courage to ask out a woman, only to be rejected. Or, he could bungle his approach due to shyness and awkwardness, making the interested party unattracted to him. This person would be both love-shy and incel.
A major tenet of continuum theory is that love-shyness and incel tend to feed one another, and people are rarely purely love-shy or incel. Many who describe themselves as incels have historically had some difficulties approaching women, while most love-shys have at least attempted to approach at some point in their lives, so there is some truth to this theory's foundation. Simple probability would add weight to its existence as well.
Of course, pure love-shys who have never approached and/or have been unable to respond to advances do exist. Pure incels who do not fear approaching exist as well. These parties tend to dismiss continuum theory.