1a. Since the Lord's purpose in sending a messenger of satan to attack his body was to
prevent Paul from boasting, and boasting is a sin, and God's will for us is our sanctification, it would make no sense for the same Lord to have sent a messenger of satan to cause Paul to sin by being filled with sexual desire.
1b. Romans 7 describes the life of a Jew (Ro 7:1) in the flesh (Ro 7:5), and being "mastered" (made a slave) by sin because he is "under the Law" (Ro 6:14), but Christians are "not in the flesh but in the spirit" (Ro 8:9) : Romans 7 does not describe Paul's life as a Christian.
2a. The "messenger of satan" attacked Paul's body--this may refer to his having had poor vision :
i. Gal 4:15For I testify to you that, if possible, you would have gouged out your eyes and given them to me.
ii. Gal 6:11See with what large letters I am writing to you with my own hand.
2b. Since it doesn't make sense that poor vision would've kept Paul from boasting, "thorn in my side" may refer to the constant resistance to his teachings (the revelations he would've boasted about) : "thorn in your side"
is a phrase from Torah referring to idolaters in the Promised Land (left alive--they were to wipe them out so that they would not become thorns in their sides), so it may refer to the "idolatry" of the churches sowed in them by the "idolaters" (eg, Judaizers : "this persuasion does not come from Him Who calls you" (Gal 5:8) means the Galatians, for instance, were committing idolatry (1 Jn 3:23 says God's command is to believe in the Name of God's Son and love others : there is no command against idolatry, but loving others is the second greatest commandment, thus believing in the Name of God's Son takes the place/fulfills the greatest command to love God and have no others gods before Him, thus veering from the faith would be "idolatry"), because they were "giving heed to doctrines of demons" (1 Ti 4:1) (and idols "are demons" (1 Co 10:20) : submitting to the false doctrine/false Gospel of a demon is
idolatry), whereby they "wickedly depart from... God" (Ps 18:21), as Paul says, "I am astonished you are so quickly deserting Him Who calls you in the grace of Christ and are going after another Gospel" (Gal 1:6)), but Paul so closely identifies with the members of the churches that he considers this "attack" on the church as an attack on his own self (just as Christ asked Paul, "Why do you persecute Me?" when Paul persecuted His Church).
That said, I'm not
entirely satisfied with these answers (2a and 2b); if anyone has a better answer, please share it.
@ChristinaL @Bruce-Leiter