Would it be illegal to have a male 17yr. and a 15 yr. old to date and have sexual intercourse if they were both willing to do so?

Yes, this would be "illegal" sexual activity. The 17 year old can be charged as an adult.

(720 ILCS 5/11-9.1) (from Ch. 38, par. 11-9.1)Sec. 11-9.1. Sexual exploitation of a child.

a) Any person commits sexual exploitation of a child if in the presence of a child and with intent or knowledge that a child would view his or her acts, that person:
(1) engages in a sexual act; or
(2) exposes his or her sex organs, anus or breast for the purpose of sexual arousal or gratification of such person or the child.

(a-5) A person commits sexual exploitation of a child who knowingly entices, coerces, or persuades a child to remove the child's clothing for the purpose of sexual arousal or gratification of the person or the child, or both.

(b) Definitions. As used in this Section:

"Sexual act" means masturbation, sexual conduct or sexual penetration as defined in Section 12-12 of this Code.

"Sex offense" means any violation of Article 11 of this Code or a violation of Section 12-13, 12-14, 12-14.1, 12-15, 12-16, or 12-16.2 of this Code.

"Child" means a person under 17 years of age.

(c) Sentence.

(1) Sexual exploitation of a child is a Class A misdemeanor. A second or subsequent violation of this Section or a substantially similar law of another state is a Class 4 felony.

(2) Sexual exploitation of a child is a Class 4 felony if the person has been previously convicted of a sex offense.

(3) Sexual exploitation of a child is a Class 4 felony if the victim was under 13 years of age at the time of the commission of the offense.