Adultery as in Judaism: intercourse between a married or engaged woman and a man other than her husband. Both are adulterers, save if she was raped, then only the man is guilty of the crime: DEU: 22:22.
Fornication as in Judaism: intercourse between persons not married to each other: both are fornicators or one (the married or engaged one, or the raping male) is an adulterer.
Note: there is no literal definition of adultery made for Christians by Jesus, thus we must interpret from what we read in his Gospel, would this have been changed for Christians. And if there is any doubt, then believers must obey the safest option.
Jesus answered a none-Jewish woman ( a gentile):MAT 15: 24 But he answered and said, I am not sent but unto the lost sheep of the house of Israel.MAT 15: 27 And she said, Truth, Lord: yet the dogs eat of the crumbs which fall from their masters’ table.
The Jews largely rejected the Gospel, but the Gentiles ate the breadcrumbs of that which was being fed to the Jews by Jesus:
the Gospel. And Jesus accepted them, even telling the Jews as a confirmation that their glory he gave to another nation (The Christians).
In the New Testament, Jesus merely refers to the ten commandments. Thus it is arguable that married males in Christianity (like in Judaism in the event of rape) can commit adultery when having intercourse with a single woman. But if God tells us to hear Paul and Paul states (uncontroversially): we are all one in Christ Jesus, then Christian man and woman are equal, and, not choosing the broad and easy way, but the narrow, believers will all refrain from Adultery and Fornication. And moreover, if Jesus tells us that his followers must be of the spirit and not of the flesh, then fornication and adultery must be shunned by all believers who prepare to respond to Christ’s call to repent and baptise in his name. Christians, however, being reborn of the spirit, are holy and thus they cannot sin and would have no such desires; and they are also guarded from accidentally courting with an engaged or married person: for they are ‘Holy’ and cannot sin. JO1 3: 9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
While the Jews largely rejected Jesus, the gentiles accepted his doctrine and adopted the Jewish laws which, according to the Gospel, applied to them: like the ten commandments and laws like fornication and adultery: the crumbs from the masters’ table.
GAL 3: 28 There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.
JOH 3: 5 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
JOH 3: 6 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
GAL 5: 16 This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
GAL 5: 17 For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
GAL 5: 18 But if ye be led of the Spirit, ye are not under the law.
GAL 5: 19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
GAL 5: 20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
GAL 5: 21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.
GAL 5: 22 But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith.