You need to think about my point more fully here. When a husband has intercourse with his already pregnant wife, is that sexual act open to its “primary” purpose? No it is not, because the wife is already pregnant. Similarly for the other situations I mentioned, such as a post-menopausal wife. Hence we can see that a sexual act that is not open to its primary purpose is not necessarily sinful. The absence of primary purpose does not equate to sin.
Hence, if you wish to establish that a specific act is sinful, then absence of primary purpose is not in itself sufficient.
Similarly, the presence of the primary purpose does not negate the sinfulness of fornication or adultery.
The presence, or absence, of the primary purpose does not in and of itself allow us to decide if an act is sinful or not. It is the other circumstances that determine the sinfulness.
You would do better to concentrate your argument on those other circumstances than on primary purpose in this discussion.
rossum