Fellatio is not prohibited by law, whereas asking a foreign leader to dig up dirt on your political rival in an election cycle is.
§30121. Contributions and donations by foreign nationals
It shall be unlawful for-
(1) a foreign national, directly or indirectly, to make-
(A) a contribution or donation of money or other thing of value, or to make an express or implied promise to make a contribution or donation, in connection with a Federal, State, or local election;
(B) a contribution or donation to a committee of a political party; or
(C) an expenditure, independent expenditure, or disbursement for an electioneering communication (within the meaning of section 30104(f)(3) of this title); or
(2) a person to solicit, accept, or receive a contribution or donation described in subparagraph (A) or (B) of paragraph (1) from a foreign national.
(b) "Foreign national" defined
As used in this section, the term "foreign national" means-
(1) a foreign principal, as such term is defined by section 611(b) of title 22, except that the term "foreign national" shall not include any individual who is a citizen of the United States; or
(2) an individual who is not a citizen of the United States or a national of the United States (as defined in section 1101(a)(22) of title 8) and who is not lawfully admitted for permanent residence, as defined by section 1101(a)(20) of title 8.
Foreign nationals - FEC.gov
Generally, an individual (including a foreign national) may volunteer personal services to a federal candidate or federal political committee without making a contribution. The Act provides this volunteer "exemption" as long as the individual performing the service is not compensated by anyone.
Donation refers to a gift to a charitable organization whereas a contribution is generally associated with a gift to a common fund or collection.