For all his failures to comprehend social context, Al's mind made up by studying and categorizing series of small changes in behaviour, scent, taste, sound and muscle tension. The same ability which allowed him to observe and understand an owl or tame an arachnid translated this variation of human behaviour to the obvious conclusion. He's aroused; he wants me.
But there was more behind Al's thoughts and actions than just the logical, calculating analysis... Al ran his hands over the sweaty skin, over the tense joints, counted the rhythm of arching hips, and enjoyed discovering every new change in Severus' state, in his movements and his voice, the rocking of his body and the taste of arousal, the hardness, the tension, the heat.
The visual or the smell wasn't enough, he wanted to touch and taste and swallow it whole, commit this moment to memory and never, ever forget this. For it belonged to the two of them, and Al was very possessive of every thing he shared with Severus.